I'm sorry OP, it sounds like a hard few days.
Just adding a note at the top. I've crammed MASSES into this post and it's far too much to do all at once - which is not what I mean for you to take on - it's basically stuff I've learned over 13+ years. Pick one or two things, or print it and cut it up and put it into envelopes with different months on to look at over time, or buy one of the books and read it with DH and try different things from it over a few weeks/months.
I do think these days around Christmas/NYE are particularly difficult with ND in the mix. DS2 (also 5 and probably ND) has been totally nuts the past week or so. Bouncing off the walls, melting down over little things. The last couple of days, DH took him out for a really long walk on one day to the next village and back, and yesterday we drove to a new park and they played there for about 90 mins - this has helped enormously and (fingers crossed?) this has hugely helped and he seems back to baseline. I'm sure we're about to wreck this tonight because we always get loads of fireworks where we live at midnight on NYE.
I think maybe it would help to separate out the issues in your post. It sounds like everything just collided to make an explosion.
So first you said you hadn't been out (at all) the previous day, this is generally a recipe for disaster in our house. The children NEED walks (like dogs). Even just a 20 min walk around the block helps release pressure. If we have had a lazy day at home, which we do sometimes, we have to make up for it the next day if we don't want to all end up screaming at each other. The more days with no exercise, the worse it gets. Christmas/winter is general is a bit of a pain for this also because they build up so much energy from all the presents and excitement overload and we (adults) don't really want to leave the house anyway. If you've had family visiting or visited family then you can multiply this again.
Secondly - I probably would not have attempted to go into town on one of those days. Town for me is a "good day" activity. It's boring for DC. They have to listen to a lot of different instructions. Be careful of the road, stay close, hold hands, don't touch that, wait, be quiet, no running. No we are not buying sweets/toys. They have to wait around while you do your thing which is hard. There is a saying which I find really helpful which is "Try hard things at easy times". So while, yes, DC do need to learn to accommodate others and come along to things which are not their favourite, it probably wouldn't have been something I would have attempted on a day that I know their energy reserves and ability to tolerate discomfort is already pretty low.
The other day we had to take both small DC to the supermarket. 5yo was creating on the floor whining that he did not want to go. We were saying unfortunately we have to go anyway (etc) - I wrote a shopping list and because we knew it would be difficult for him we basically planned for it to be a quick trip in and out. I can't remember exactly what the sequence of events was but he basically said can we go to <this specific supermarket> and I said yes, OK - because that way, we had some buy in from him and that helped him to be cooperative, and ultimately for us it didn't matter what supermarket it was, as long as we could buy food. What I might have done if this hadn't been suggested would have been bribery basically - I might have started talking about some aspect of the supermarket which I knew would be appealing to him such as suggesting that we buy some sweets, ice cream or have a look at the toy section or suggest that we look for his Elsa shampoo or that he can choose what he wants to eat for lunch (it doesn't always have to be some overtly "treat" type thing, but I might break that out on a really hard day)
If I was going to separate out two more parts of your post I'd say:
DS is not your sister. I totally get looking ahead and seeing that scenario playing out again but it seems like you're seeing two possibilities: Give in and let him "dictate" or lock horns and fight. I think there are myriad approaches in the middle.
The locking horns and fighting approach: I will out-stubborn you. Again I can see why you would try it - but it does not seem to be working for any of you, and I think honestly it will ramp up his distress and make things worse.
Middle ways would be:
- Trying hard things at easy times.
- Breaking down expectations into chunkable steps - coming into town (busy, people, music, lights, smells, boring, instructions), which he doesn't want to, on a day that he's dysregulated and hasn't had exercise the previous day, when he feels forced, and you're dysregulated - this is SO much - you want to work on each of these goals separately and in small steps. There's a great phrase called the "growth zone" - basically everyone has a comfort zone, where they feel totally comfortable and safe and don't have to do anything hard - letting DS stay in his comfort zone would be tiptoeing around and doing everything the way he wants all the time. But you try to drag him too far away from his comfort zone and he'll be in the "danger zone" where he's going into fight or flight, running away, hitting you, shouting, lying on the floor, refusing to do things etc. The growth zone is small and it exists between these two. You don't get growth by forcing it, you have to nurture it. So for example attempting the supermarket on a really good day with lots of supports, when it's a trip that he wants, to attempting the supermarket on a good day where there are things he wants but also things other people need, to a trip where there are lots of supports but the trip is not centred on him, reducing supports over time.
- Setting yourselves up to succeed - try to "top him up" with lots of regulating sensory input. Get his buy in for things (approach with an aspect that he will be interested in). If he struggles with sensory input, whether overtly or just more of a pattern you've observed, then you could explore things like ear defenders and/or sunglasses for the supermarket. Let him sit in the main part of the trolley, and watch videos on a phone with headphones (and ignore any judgy looks - who cares, if it gets you a safe trip out). Give him a job within the supermarket such as reading the shopping list, carrying a basket or a child's trolley, finding specific items, doing simple maths sums etc (I just read yesterday that when they are heading into meltdown territory, some kind of cognitive task, such as reading, guessing, searching, or numbers can help to "bring them back" into the logical part of their brain).
- Empathise when he doesn't want to do something (even if you have to do it anyway). Seeing his point of view (REALLY seeing it) goes a long way.
- Problem solving - seeing if you can get aspects of your concerns and his concerns and work out a win-win solution. For example, you needed to go out, but he didn't want to go to town. Was there anywhere he did want to go, would that have worked for you?
- Separate your concerns/agenda from his. I think your DH is right - if your immediate and urgent need was to get out of the house, then you could have prioritised that and gone with DD, putting that over the need to get DS to be more flexible on that occasion. (Not forever). It's true that you won't get flexibility by accommodating inflexibility all the time, but you also can't force it as that will just cause him to tense up and push back. It makes sense to put your own oxygen mask on first and see to your own needs separately from trying to help DS cope with situations where he has to do things he doesn't want to.
- Positive reinforcement - apparently the ideal ratio of praise to correction is 4:1. Children with ADHD or ASD frequently have this ratio the other way around - they receive something like 10x more corrections than praise. It's really shocking and probably contributes to dysregulation as well as leading to scenarios where they're basically ignoring being told off/punished because it happens to them so often they just accept it as an immutable fact of life, also cementing an image "I'm a bad boy" which they then lean into. There are some really really good methods to increase the amount of praise you're giving which will help flip this ratio to something where it's actually constructive.
- Pinpoint a small number of current problem behaviours and flip them into positive expectations e.g. "Not hitting sister" could be "Play nicely with sister". Break this down into what it actually looks like in terms of actions e.g. "When you want a toy from sister, offer her another toy". or "Using nice tone of voice when asking for things" Write these down. Keep it to a small number. Because they are written down you can look for them and when you see it, you can praise, when you might normally not register it at all. If you realise it is actually unrealistic, change it e.g. "Ask adult for help when you want toy from sister".
- Consider a points/reward chart/marble jar system where you can reinforce praise with a small reward.
- Stick up smiley face emojis around the house. When you see one, use it as a cue to say something positive (if there is nothing positive to say, say "I love you")
- Try to "catch" him being good even if it is the smallest thing like having got dressed or eaten without making a mess.
- Every time you notice yourself telling off/correcting, look for 3-4 reasons to praise over the next few minutes to help balance.
- Praise for behaviour which is 25% correct. This is more motivating than waiting for 75%+ which might never happen otherwise.
- Look at positive parenting resources - Triple P course may be available in your area, 123 Magic has some of this, ABCs of Everyday Parenting on Coursera (free) - bear in mind that some of the tools offered in these courses can be ineffective if the child CANNOT do the desired behaviour. (Which might be the case even if they can do it sometimes). These tend to have really good guidance on how to give effusive, specific, tactile praise (which is more effective than "Good boy!")
- If your child struggles with effusive, direct, tactile praise try just noticing without appraising "I noticed that when you wanted that toy, you offered sister a different car." (NOT that was good, or well done, or good boy).
- Praise for individual parts e.g. if you want to stop a behaviour where he wants a toy so he screams and hits and snatches, break this down into "not screaming" "calm voice" "using please" "not name calling" "waiting to be offered toy" "not hitting" "asking adult for help" "offering sister alternative toy" - and though it will feel counterproductive, if he does any of these e.g. he shouts PLEASE GIVE ME THE TOY - praise for using please, even though he shouted. Or if he does not shout but has a calmer voice, even if he bluntly demands "Give me that now." Praise for not shouting. When you do this technique it will feel really tempting to add a critique about what he got wrong, but try to avoid this as it cancels out the effects of the praise.
There is this FANTASTIC book called When Your Kids Push Your Buttons which would probably be useful to read. It's not diagnosis specific but it really does help you figure out where some of these feelings are coming from and it helps with approaches which are likely to be less bull-locking-horns.
The Explosive Child by Ross Greene is also really good. Each of these have approaches for combining the child's agenda, needs or concerns and the parent or other family member's agenda, needs or concerns.
Also I would recommend following "The OT Butterfly" on instagram and The Occuplaytional Therapist on Facebook (and look at both their websites) as they both have really really fantastic accessible info about sensory regulation and dysregulation and how to handle children who struggle with demands.
Lastly - have you ever spoken to your parents about the way your DSis was as a child and the way you struggle with DS? I wonder if this would be a helpful conversation to have. I wonder if perhaps they also tried the locking horns approach and found it unhelpful, and this is why they went with the let her be approach - which might be useful information for you, to help put in context things that might have happened as a child which didn't make sense to you at the time, it's possible there was more going on behind the scenes that you weren't aware of. You could approach this as being worried about the effect on your DD. How is your DSis now as an adult?