Parent to an adhd/ASC teen and preteen here. I agree that some of this may be about parenting, but parenting send children is relentless at times.
Have you done the stepping stones and cygnet courses? I found them very helpful. Not least because there were other parents in the same boat.
Equally, why have you come off your meds? I am on fluoxetine and find it much, much harder to cope if I accidentally miss a dose. So while on them, I don't necessarily notice the effect, but I do when not. If that makes sense. Maybe ask about a different medication. It doesn't sound like coming off them is helpful at all right now. It's maybe not so much the veil has been lifted but your coping strategy has been removed. I genuinely cannot come off my medication. I've been on antidepressants 13 years now. It took a while to find the right one and the right dose - with this one I am on the lowest. But I need that strategy for me to be able to deal with everything life throws at me.
Then: set boundaries. Your son might be acting out because he does not feel 'safe' and is trying to find where the boundary is.
Set a routine and stick to it. Including your son in making that. Make your bedroom a no go zone so it can be your space to chill and regroup.
All kids, Neurotypical and neurodiverse, need rules and sanctions. Ban bloody Roblox if you have to - ds1 is not allowed Fortnite. He turned into an awful teen on it and within a couple of days of banning it, I had my lovely son back. Instead he monologues about Pokémon, but I can cope with that.
Make consequences swift and related to the behaviour. So spitting in the hot tub means no hot tub for him for the rest of the day if that's a non-negotiable for you. Eating and leaving food/rubbish means eating has to be done at the dining table only and so on.
Does he understand why grandad is no longer around? Does he understand why that has made you sad?
It seems like a lot is going on in your family right now and