Bubbles, welcome to the forum. My heart goes out to you! I know a little about what you have been going through as my own daughter is in jail right now and her behavior was violent and she seethed with anger for a number of years.

I've done some interesting research about the development of the human brain from birth until about the age of 20-23. The interesting thing is that the frontal lobe of the brain doesn't fully develop until the early 20's. The frontal lobe is the area of the brain that controls impuplsiveness and the ability to make wise choices. They lack the ability to discern long term consequences. We can basically "hold them to the line" behavior wise, but until that development happens and the training plus the maturity takes place, they are loose cannons.
It sound like you have gone way beyond what anyone would have done and are the stability the boy has needed.
Any chance he can escape?? Please guard yourself and your family. If he is that angry and full of rage, he really is lethal.