Who has "drummed into you" that he's "sensitive" ? Just dh or his ex/sons mother too? Does she discipline him and he takes it and only plays up for dad or are they both spoiling him? If it's just dad and his Disney dad crap his ex is probably tearing her hair out too! She might even prove an ally as it would very much be for the benefit of her too if he cuts the Disney dad nonsense out!
If it's both of them that's going to be harder to manage as you can't control how he is dealt with at home, you can though make clear that different houses have different rules he's plenty old enough to understand that
is DH's fault (and his exes)
That's going to make it really difficult as dh won't want to be the "bad guy" or risk his son not wanting to see him when he hits awkward teen stage
I agree with majority of pps, I'd have instantly and seriously told him off too!
I've looked after and disciplined (when necessary) lots of "other people's kids" I think there may be an element of generational difference as I was raised that neighbours, parents friends, as well as aunts uncles and grandparents absolutely had the right to tell us off!
And if we went running to our parents crying that we'd been told off we'd get ANOTHER telling off for the bad behaviour and "making a show of us" too
Personally I think we need to get back to that setup! We'd have a damn sight more resilient kids then!
As you have an older child that's not dhs I'm wondering did you not notice the difference in how dh treated them both? Because surely that's an indicator of his unbalanced and unfair approach to parenting?
Anyway, you are where you are now.
In your shoes I'd be having serious words with dh BEFORE dss' next scheduled contact at yours (when is this?) and make certain things clear
1 DANGEROUS behaviour will not be tolerated from ANY of the children and that very much includes his son
2 ALL the children are to be disciplined and sanctioned in the same way for the same infractions with only age being a SLIGHTLY mitigating factor. Crocodile tears, sulks and similar will NOT only not get them out of a sanction it will lead to it increasing. I was a single parent, if dd protested a sanction or worse made the kind of fuss this kids making I'd have increased the sanction each and every time she didn't stop! She's no angel and nor am I but she's a far more resilient kid than many of her peers. Her closest 2 friends though had similar parenting and are similarly pretty resilient.
3 HIS (dh) sulking needs to stop now! It's childish, manipulative and controlling and recognised as a form of emotional abuse in adults.
4 IF his son continues to behave in risky ways then you WILL be deciding that in future dh needs to have contact away from your home. It's not ideal but safety of smaller dc especially HAS to come first
He (dh) needs to grow the fuck up!
I recently came across this article and thought it was pretty good might give you some idea how to approach him?
https://fathers.com/featured-resource-center-page/dont-be-a-disney-dad-guest-blog/
Or you could even send him the link when you think he'd be most receptive.
As a single parent when dd still saw her dad I had the same problem as in her dad just let her do what she liked and he didn't care for her properly and she came back from contact with her head all over the place! I'd then have to pick up the pieces of a confused, frightened, exhausted little girl usually an upset stomach too (combination of anxiety, lack of sleep and too much crap food at crazy times!) and at times she'd lash out.
Kids NEED boundaries, they may not like them "in the moment" but they make them feel secure, confident and loved.
On another thread recently I was talking about my dd having more freedom as she got older (into teen years) yet her peers at the same age who's parents hadn't enforced boundaries at a younger stage were going a bit wild and their parents were tearing their hair out and TRYING to rein them in, it was very much a case of "too little too late" dh REALLY needs to get a handle on this soon or when puberty hormones start kicking in you're going to have even more difficulty especially as it's a boy you're dealing with and they need to really control especially "violent" impulses.
Dh is really doing him no favours long term.
And alllll that is without even CONSIDERING how your dc are going to feel seeing this child getting away with (almost literally!) murder and they're getting a bollocking for as they will likely see it "the slightest thing" that will breed resentment and serious dysfunction - it's essentially the extreme golden child and scapegoat set up! And I'm betting your eldest will be the main scapegoat! Just read the "stately homes" threads on here to see the adult ramifications of that!
https://www.google.co.uk/amp/s/www.theodysseyonline.com/amp/7-reasons-why-being-the-golden-child-isnt-so-golden-2478390217
See point 5 in particular
The bullying point is a good one! Other kids won't tiptoe around him and if they sense his "weakness" it'll make him a prime target!
One of my dn's was like this, the messed up parenting admittedly started as they were premature snd very poorly for the first few years of life so understandably they were over protective, but when they started primary school his lack of resilience was noticed by the school (would cry even if asked to answer a simple question eg when going around the class answering on times tables) and brought up with the parents.
Parents initially very defensive but myself and other relatives, hopefully gently basically said we could understand why they were upset BUT the school was right. We'd tried raising it with them before to no avail and I certainly didn't treat them differently if I was watching dd and her cousins but I knew the likely response and "jollied" them out of it AFTER they calmed down so not a reward or placatory.
What's interesting is despite my being the "strict" one out of their parents' siblings they loved coming with me and even asked for me to be the sitter/minder when given a choice. I HOPE it's because I am also lots of fun and that my love for them is what they most feel from me.
How do the other dc respond to dh telling them off?
His nonsense that he treats them the same - I'd be sorely tempted to video him telling your dc off and telling his off so it'll be harder to deny the stark difference!
Interesting idea almost mooted by a pp - if he thinks how he "disciplines" his son then he "disciplines" the other dc in exactly the same way? I'm not suggesting you put this into practice but posit it as a suggestion to him and see how he responds, my guess is he'll try and say it's not good enough or stringent enough - well if it's good enough for dss...
In fact one of the main ways he seeks attention and validation is by demonstrating how good he is.
Again - see advice and research on the damage being the golden child does. Are there other dc in his mother's home too?
This kid is SCREAMING out for boundaries and to properly feel part of your family.
DSS has another sibling at his mum's house, also toddler age.
I'm also not qualified to even guess at potential SN but I do have some knowledge of child psychology in neurotypical children and while I'm not ruling out SN I think just plain poor parenting and dysfunction is likely the cause.
I think family therapy would be a good start actually.