ADHD kids are about 30% behind thier peers in maturity. This, combined with impulsivity can interfere with social interactions. Extra coaching can help and placing him in a position to help younger children can build confidence (they tend to do best with younger kids). My youngest helps younger kids with school and homework and helps supervise the younger kids on the playground.
If you have a children's hospital or university nearby with a child development department, you can see if they have social skills classes or other helpful services.
The meds may need to be evaluated again to make sure he has max benefit for impulsivity. It may be that something like guanfacine would be helpful. My youngest is on this, even when the stimulant is not in her system. She had severe anxiety and depression due to school failure and social issues, as she has gotten older (and social coaching and effective meds), she is much better with her peers and has friends.
My son with add (7 years old) has horrible times socializing. He tries in his own way to make friends but he's just different. He has a hard time reading social cues and I think he's been rejected so many times that he now has a hard time trying to make friends. He's getting very anxious in social situations (lunch, recess) and introverts himself in them. (he is medicated with zoloft for anxiety too) If he does socialize at all he usually does things that seem stupid and inappropriate (act like he's unconcious or yell stupid things like his dad likes to fart or something completely random and dumb). I've talked with him several times trying to help him understand how to act or at least what NOT to do. He's a really sensitive kid. He's the nicest boy on earth. He's just trying to be funny like a class clown in hopes of attracting friends and I think its backfiring on him because his humor isn't matched with what the other kids think is funny. He's afraid to "be himself". anyone have any tips on socialization problems or making friends???Hi, I'm new to this, but I did have a thought. Have you done behavioral play yet? From what I've heard, perhaps a counselor or social worker can work with your son to learn the desired behaviors of making and keeping friends. Good luck to you.There are social skills classses given by therapists who specialize in ADHD. You could ask your ped for a refferal or if you have a psych yet ask them. They can be very helpful with social situations. Is he medicated other than the zoloft? Mine is starting it today. How are the results?All the therapies above are really helpful. My dauighter is in 7th grade and still does a weekly coial skills group. They work on eye contact, listening, taking turns in conversation...etc., etc. I read a helpful book a few years ago.
Good Friends Are Hard to Find: Help Your Child Find, Make, and Keep Friends by Fred Frankel and Barry Wetmore
It gives you suggestions on how to help the child make friends. There is a whole section on children with ADHD.
My son 9, also lacks social skills. He plays with the younger kids on the playground and our school has a mentoring Big Buddy thing where he mentors younger kids. He loves it! There are social skills groups that we will be attending once his meds are settled down. They are run by his psychologists office. I notice most of the boys are a little older than my son. So I am not sure when he will do it. Right now we are working with a psychologist on getting him to take his meds and lower his anxiety.We seem to always have social problems. I feel that it is the clickiness going on at young ages and kids are just plain mean, adhd or no adhd.
My son also is now quiet, minds his own business. He just gets through recess and all. He joins in, but is really not included because he is not in the "click." And the click is from their mothers click. Small town stuff. It is so competitive. I feel sorry for the kids these days. All this unnecessary pressure put on them by their families.
What I did with our son is took him to close by towns and joined their activities for him to meet other kids, just feel included in something. It is an on going challenge. I just had a conversation about this with him the other night. he told me he is ok over at school but it does bother him more than he tells us. 
You need to find a place for your son to be himself, try your Rec department, or YMCA, leave your town and go to the next town over. Find someplace where he can get a fresh start, no one knows him but can get to know him! 
Your son needs this for his self esteem!!
We've had a hard time with this as well. My son is 8 and I remember at his 5 yr wellness exam the Dr. asking if he had a favorite friend. I know he likes to play alone - he gets intense about whatever his activity is. But we've tried soccer, T-ball, and Scouts, and he has a very hard time in groups. In fact, we are thinking of pulling him out of scouts. I'm thinking now he needs all one-on-on play dates and lessons. We visited a dance studio tonight and I am trying to decide if it's worth a lesson to give him private dance lessons. I would suggest finding another child who doesn't quite "fit in" and set up play dates. My son does well with his sister because of the familiarity, so it might be good to help establish that with a friend. I think for children who dont do well with tam or group sports, individual sports are great. They still do things they can enjoy without "having to fit in". Examples are swimming, horseback riding, gymnastics, karate or tae kwon do, skating(not hockey)....like that. This gives an interest and something to enjo without really having to fit in the mix. They meet kids, but dont have to really blend. My daughter has made some good friendships with her developed love of horses. Shes not a great rider even after almost 4 years of lessons, but it gives her something to talk about and she loves it. Great way to make playdates with children outside your school also.FYI - I have mentioned this before - some boys just don't have friends until later. My husband, who has no dx at all, says he really didn't have "friends" until he was in 3rd or 4th grade. Also, when he did make friends it was only a few during elementary school - by the time he was in high school he had ton of friends.
My son took horseback riding over the summer and loved it! Maybe I'll give that a try. We live in the suburbs of Los Angeles but there are many place to ride on the outskirts.