My daughter was diagnosed *severe* ADHD, but also anxiety and OCD. She showed aggressive, very defiant, and very impulsive/inattentive behaviors. One thing I have learned in that people with ADHD can feel stress more easily and quicker than others, in part because of the way their brains work. It's not so easy to cope with even simple every day stress because more stress gets created from the effects ADHD has on that person, and with it, creates even more frustration. I hope this explanation from my view doesn't confuse you. Still, like the others said, aggressive behavior should not be accepted and should be brought up with his psychiatrist, assuming he has one available. If not, I do recommend it. Medicating for ADHD and any other involved disorders should make an extreme difference--he (and you) will be glad!