An excellent place to start is to contact the national office of Children and Adults with Attention Deficit Disorders (CHADD) at 1-800-233-4050. They can put you in touch with a local branch of this parent advocacy organization that can help you navigate through the school system to get what you need to support your son.
You might also want to read Harvey Parker's Problem Solver Guide for Students with ADHD. He gives a good list of possible Section 504 accommodations you might want to request.