The day after my last post, I read that Adderrall can cause irritability. We took him off of it (and the Risperdal) immediately and the violent tantrums all but vanished. After a week, we went back to his pediatrician and told them we wanted to try a nonstimulant ADHD med. they gave us a starter pack of Intuniv.
He's been taking it for two weeks now, and the violent behavior has not returned. Unlike the stimulant meds, Intuniv takes a while to start working, but I think it's helping. We need to talk with his teacher and see if she thinks this dose is helping enough.