Here is something for you to think about.
Go to some local martial arts studios in your area. They are usually pretty quiet during the day. Tell them you are thinking of enrolling your son because of, and then tell them your problem. Speak to the instructor(s). Tell them you want your son to be able to defend himself, but you also want him to not be the bully, but to be confident.
They may be able to help you out.
Judge Dread