Life, he's running right now and fast. Everything about your life together feels like a trap that is trying to consume him. He believes he's entitled to happiness before he dies and he views responsibility to you and the kids as a deterrent to that happiness.
Taking on the care of someone else's kid is a choice for him. Not something that is expected of him but a free will choice. He also has no financial obligation for the child. (The kid doesn't feel like a trap to him) Everything he does for the child is probably heralded as him being a hero because he really has no obligation to do it.
Real life has a way of rearing its head after a while and they find out that the fantasy life they thought they were living turns out to have the same expectations of him as before because that's life.
Until he realizes his happiness comes from within himself and really starts to deal with his issues, he'll continue to run.
All you can do until that time comes, if it comes, is do exactly what you're doing. Working on yourself, and protecting your children and yourself the best you can from his path of destruction.
Continue to do that and you'll be better than OK no matter which way your sitch turns out.
