This is going to sound really insensitive, but I don't mean it that way.
i remember that you have a somewhat friendly relationship with your abusive ex-husband, but your ex-husband doesn't want much, if anything, to do with your son.
If I was going to play amateur psychiatrist, which I do way to often, what you just said sounds like your ex's abuse was as much of a trigger for your son as genetics, or drug abuse, or anything else. And, since I know drug addicts (a lot of them), I'm going to guess that he does drugs so for just a little while he doesn't have to remember - and perhaps it's that abuse he doesn't want to remember or can't forget. Watching someone you love get abused & not being able to stop it can be way worse than getting abused yourself.
Have you ever pointed out that to your ex-husband? That it might all go back to that? Or, would he think that's all a bunch of BS.
Heat could even represent your ex husband.
I'm sorry if that sounds a little rough, but I find the distinct personalities, especially related to your history, fascinating.
I also think some trouble in my marriage early on triggered at least some of my son's illness. There wasn't much of anything physical, but a lot of verbal fights - and I was just as bad as his father so I'm equally to blame. Not that it makes it any better. So, I'm not pointing any fingers that I wouldn't point at myself, just a little curious.