So you did get legal advice about the steps to take.
What were possible solutions offered by all the therapists/treatment centers he has been to? Surely they have made suggestions.
Has he been seen by a psychiatrist, tested for mental illness - does he need prescribed medication?
Is there a better, or more long term residential facility he can go? Have you researched all of these avenues?
I am responding from the point of view of reading similar posts...also from legal parents, however the biological parents...but you are his legal parent, like them. There are others going through this and it sounds like h*** and I have even seen it recommended to find a treatment place to take the one potentially violent child out of the home if there is fear he will harm the family. As a parent, that is a mature choice. However, I think you have to do what these other parents did...muddle through this...find residential places for son, make sure he is thoroughly tested...I can't help but wonder if a correct mental illness is assessed and medications tried.