Yeru - just let him take his time. He's an adult now, and his upbringing sounds like its been emotionally upheaving & confusing at best. Is he going to college?
Getting him adjusted to "normalcy" in itself will probably take awhile. I hope his younger siblings are okay. They'll probably be coming along shortly.
As far as his religious views, give him time. Let him weed it out for himself. Its obvious that his father has pretty much "written him off", by not even offering to come to take him to meetings. What Christian love!!!!!
I think you're great to take on such a complicated situation.
God Bless You and Your Family.
Peace
Rhonda