Fight I'm sorry that you are going through this. I hope that you and S are safe in the home. If he is still living there it is probably best that you don't discuss the divorce or relationship with him at all and try to de-escalate the situation as much as possible.
These are his choices. Please don't beat yourself up but i'm glad you are coming to terms with why you put up with him (I still haven't figured that out in my own situation).
Your S will be fine and the time you can focus on you and healing will make you that much stronger and present when you come back to him. Keep in mind that just because he may get 50% custody, does not mean that he will exercise it. Your S is younger, but my kids said upon learning about the OW1 and my H telling them he was leaving me, "at least there won't be a custody battle." They knew their father was very disconneccted from them.
I bet the more you can make him believe that you are ok with him having S, the less he will fight you on it. It seems to me like he really enjoys pushing your buttons in a very sadistic way. Walking around behind you criticizing everything you do is much different than actually having to parent a young child on his own. And that time will take away from the swinging bachelor life he is probably imaging for himself.
