P.S. I have brought my son to several child psychologists and I have been to many different ADHD classes, seminars, etc. -- to no avail, it seems.
In order for your son to pull out of this, he's got to have something outside of school that is more attractive than school or those tough kids, or other bad influences. Some students find it easier to resist the temptation to do the wrong thing when they get involved in scouts or a church or temple related youth group. It would be very good to find someone who can serve as a mentor or guide for your son. The right teacher may just be able to play this role. Finding your son a Big Brother, a service which may be available in your area may also be a good way to connect your son to better influences. It sounds as if he is too young for a "real" job, but having a chance to work in a family business or in the school (perhaps as part of a work-study program) would add meaning and purpose to his life and help keep him on the right track. Programs such as Outward Bound, that engage teenagers in safe but challenging encounters in the out-of-doors, might help your son get in touch with his inner strengths and develop a better self-image.
A consultation with a psychopharmacologist who specializes in the treatment of adolescents can help you determine if your son is on the right doses of the right medication. You can ask your pediatrician for a referral or call a children's hospital in your area. Since you and your son have gotten into some struggles about his choice of friends and activities, you might think about seeking the services of a family therapist to help you communicate better and work out some mutually agreeable guidelines. Contact the school guidance counselor for a referral, or call the state affiliate of the American Psychological Association (www.apa.org) in your state capital. A group at school run by the guidance counselor or school psychologist may also be a way for your son to form more wholesome relationships with other children.