You will want to post a copy of this post to the Special Education board here at CD. A private school that doesn't specialize in difficult children doesn't sound like a very good placement for this boy.
I would advise that this parent consider having this child evaluated by a neurologist for issues like seizures and other neurological issues that could be mimicing NonVerbal Learning Disorder (NVLD) or compounding the problems. Sometimes when behaviors are very aggressive like this there are other issues going on that are being missed because there seems to be an explanation.
Have the parents considered medication treatment? If all else is failing that would be a legitimate option to explore.