my son is almost 12 but he has autism and adhd so I understand where your concerns come from. It is not easy being a special needs parent. One thing that has to happen is a strict routine, when we mess even slightly with my son's routine it is a disaster. I don't think sending him off to his father's when he is violent is the answer, these kids have very short attention spans (even when medicated) and he will not understand why he is being punished. It is better to remove him to a safe place where he can calm down. Your fiancee and her son definitely need family therapy as the situation right now is out of control.

Hope that helps.