Crisc, so sorry you are all going through this turmoil!
I went hunting for a NY Times article published about 3 years ago whereby the issue of aggression in children was explained. In the article, which I cannot now find, darn it, aggressiveness was said to be a normal part of development, especially a child "trying out" the role of domination over someone else.
However, as I was doing my hunting through NY Times articles, I read a few other interesting tidbits. One, which applies to bullies, which I believe is NOT the case for your DS, points to communication issues with parents. Specifically, this article, published in 1988 (I know, ancient), said bullies' parents used physical touch for discipline only. Physical touch for affection and praise, plus lots of verbal praise and affection, were not present in a bully's life. Again - not your DS's situation but I mention it in case extra doses of cuddling and verbal praise might possibly help.
Another article, published in 1989, outlined aggression in otherwise docile children. It says the aggression is misplaced frustration. Here's the link to that one:
http://query.nytimes.com/gst/fullpa...p=6&sq=aggressive%20child&st=cseI know you didn't ask for old research, but I thought I'd post this anyway. I'm sorry I couldn't find that article I went looking for!
All the best ~ Mary