Is your son on medication? Have you spoken to the doctor or a child psychologist about his behavior?
Possible things to consider: reaction to med, food ingredient (switch to potato bread for a while), dosage, what outlet for energy does he have, what is his behavior in school, social interactions changed any?
Does he enjoy LEGOS or Knex? Something that would keep his attention creating. ADD will stay focused on something they are interested in. I don't know much about the HD part. But it sounds like you really need to check back for a medical assessment. If one doesn't help, get another. Consider how frustrated he is that he can't control himself. He knows he doesn't feel or act right but cannot verbalize or understand it anymore than you do. You need to look at this from his perspective. Do research and get help for both of you.
What about taking up biking together, getting him in martial arts (they teach discipline) and help focus, track, soccer or some relatively continuous sport activity. Baseball would be out as there is a lot of sitting and waiting. Tennis or swim team would be good.
polystyrene is great for braking and getting your frustrations out, but it makes a big mess. i suggest finding hobbies such as sports (individual or team) or art, i enjoy painting, music or what ever he seems to find enjoyable, this will give him an outlet that is productive. I know it is harder than it is sounds but do stay positive (when ever possible) and calm as i am sure it is hard for your son as well. Maybe talk to his school councilor and see if they can give any more ideas and support you and your son.
Best of luck
hiya,
maybe he can rip up paper, he is looking for an outlet of some kind, maybe exercise or something. its hard when his behaviour is so difficult but u need to keep going and not get frustrated with him. just stay as calm as possible.
hope your ok!