Is there something at school or home that is frustrating him (a bully, a teacher he doesn't like, a new baby)? I would try to figure out the source of the anger, like Doolallygirl says.
However, I think you should be wary of teaching him that anger is unacceptable... boys in particular in our society are taught that, and then when they don't know what to do with their anger, it manifests in dangerous ways. Much better to teach him that anger is okay, but there are good ways and bad ways of expressing it. Saying to you "I'm angry at you" is a fine way of expressing it; throwing something at you is not fine. When he's angry, you could try suggesting ways of working through his anger or redirecting it that are acceptable, like talking to you about it, drawing about it, dancing or something else physical.