Have you spoken with your school guidance counselor or psychologist about the problems your son is having? Did a child psychiatrist prescribe his medication? If so, that professional should also be consulted. It's important to know the reality of the situation in school. Have you had a conference with your son's teacher? Is she seeing the same bleak picture your son perceives? If the problem is not really with your son but with the other children in his class, that needs to be addressed directly in school.
How do you see your son relating to other children in your presence? If the problem is with his social skills and how he relates to other kids, there are programs that can help. For example, the Skillstreaming series available from ADD Warehouse (http://www.addwarehouse.com or 1-800-233-9273) is an excellent program to help children develop and practice social skills. Role playing social situations can be really helpful. Play out situations where your son feels isolated or rejected by others and practice positive behaviors he might apply.
Is there an area of strength or special interest that could be cultivated in your child to build up his self-esteem? Dr. Robert Brooks, a wonderful child psychologist, writes about the need to help our children experience success by identifying and reinforcing their "Islands of Competence." When kids discover their strengths, they are more willing to confront even those areas that have proved to be challenging to them. Look at Dr. Brooks' excellent book, Raising Resilient Children: Fostering Strength, Hope and Optimism in Your Child.