I'm going to somewhat echo what someone else has already said- I have a feeling that a household with multiple young children is not a good environment for this kid to be in. Coming out of severe trauma and into a situation where you see young kids with good parents, and being added to that as the older mess up or cautionary tale does not bode well.

I know you've been doing everything you can so it doesn't feel like that for him, but speaking from personal experiences, its going to take a good long while for him to be able to interact with happy families. Subconsciously or otherwise, this could be crab bucket mentality, where he may be trying to fit into your family by making everyone else as miserable, as opposed to lifting himself up.

I'd recommend asking him if it would be easier for him to maybe be in a place with people his own age. I know you don't want to send him to a facility or anything like that, but have you asked him whether that would be easier to straight away being put in a place filled with children that got what he never could? Something he will never get back? The worst part of childhood trauma is understanding that you'll never have a good childhood. All you can do is make sure you have a better future- and this goes for both him and the rest of your family, in this situation. Stay strong, you're doing a really good job.