So very sorry about this Sarah, I wish I had something good to offer you but I only have this... until you stop him, until you set the boundary that he just can't do this and expect to also have a W and a home, he isn't going to suddenly stop. Every time my W started to make a positive change, something new would happen (in her head) and it was right back to me and her bad M causing her pain. The more "space" I gave, the more she needed. You say he is staying for your S's sake? How is acting the way he is now good for your S? How is treating you the way he is helping things? He seems to respond when you don't allow him to get away with crap, when he notices that you are pulling away. Pull away totally. Don't respond to his crap. Don't get drawn into his drama. How can things get worse?
Like you said, he is most likely going to leave anyway. Let him go and GAL while he is away. You are going to be fine. You are stronger than you think. I can tell from your posts.
