If he's just hitting himself with objects, I wouldn't worry too much, but if he's head-banging on the walls, floors, tables, or other hard objects, he could do brain damage and it could be due to neurological pathway differences, such as in Autism. Head-banging is just one of the symptoms of Autism. I suggest going to your pediatrician and Superintendent of Schools for a complete neurological evaluation. Be sure to mention the possibility of Autism! Sometimes pediatricians don't think of it until parents mention it! Follow your gut instinct despite what doctors may say!!! They often discount mothers' concerns, even if they (mothers) are professionals. Early diagnosis is CRITICAL for a positive outcome. My 35 yr. old son has it, and although he didn't get a definitive diagnosis until he was 13, due to being fairly moderate- to high-functioning, he did receive special education all the way through school to treat his symptoms, which was critical to his progress. He is now going to Bakersfield College, taking a course for a certificate in Computer Business Systems, with help from the Disabilities Services Office there.
Contact the autismsocietyofamerica.org website for more information on Autism. There are many other organizations for parent support, too. In Bakersfield, we have the Kern Regional Center for the Developmentally Disabled, which does free testing, Kern Autism Network, Hearts Connection, and others that put on workshops and conferences to provide parents with information to help them.