Well, because ADD does run in your family it is wise to learn about it. For your child, the symptoms you mentioned are not really specific enough to say he has AD/HD. Two very good online resources are mentioned below:
You also might try posting in the child behavior forum to see if they have any ideas on ways to help your son deal with his anger and frustration. That's probably one place I disagree with your doctor. I don't think head butting, etc. is normal at that age. Something must be triggering his frustrations, and that is worth looking into. The other posters on the child behavior forum might be able to help there. Also its worth looking at some of the other posts (and answers) by parents to get a feeling for problems that 3 and 4 year olds have. If you do still have concerns after more research, I would look for a different doctor. Probably a pediatric psychiatrist or psychologist.
Hope this helps. Please feel free to post if you have any more questions.
By the way, I also should have mentioned sensory processing disorder. Check out this site for info. http://www.sensory-processing-disorder.com/