Yikes. Sending you a hug.
What's his mother's attitude toward the situation? Is she denying, enabling or does she get it? If she gets it, maybe talk to her about what you're going through and that your main focus needs to be on having a healthy environment for your children?
That may be a bad suggestion if she's in denial about her baby boy, but since it seems to be the main stumbling block for you getting out of this situation, addressing it directly might be worth a try?
He is one piece of work. I keep hearing the Beatles song lyric in my head when I read your posts, "All through the day, I Me Mine, I Me Mine, I Me Mine." That's this guy.