Help, 9 year old son with self esteem issues, diagnosed two years ago with Add. Please post suggestons:)

We have a son who is 9 and we always knew since he was very young he had great difficulty following the easiest of directions. since 1st grade he has been formally diagnosed with ADD. It was a difficult decision, but Ritalin is doing wonders for him school wise. My lingering concern is his lack of self esteem. I've recently come to find out he stands alone on the playground for every recess on a daily basis. This breaks my heart. He also has asthma and is anaphylactic to all nuts.. i think he feels different from other kids due to these differences. We spend lots of time with him and I tell him that he is a worth while individual and it important to seek out friends outside of our home. Lately he doesnt want to seek out other kids in the neighborhood, not like he used to. We also have a son who is 7 and they get along well.