My 5 year old has been diagnosed with Social Pragmatic Communicative Disorder. It's a newer diagnosis, but think Asperger's without the specific traits of Autism. He has trouble understanding social language and understanding how to approach other kids. He is very comfortable playing by himself, but he does want to be included. He sometimes struggles with how to do that.
The think that has helped him the most is going to "Social Skills" at a local agency. Also, he does receive speech services at school. They were just for articulation at first, but as his speech has improved his speech teacher has really shifted the focus to include social skills. He is making great strides!!!
Maybe talk to your pediatrician to see if there is an agency near you that offers something like this. It is often something that children with Autism will receive, so you'll want to see if an agency can take clients who do not have that diagnosis. Still a good program!
Another thing you can do is talk to his teacher about services through the school. My son's kindergarten teacher asked us if he would see the school counselor for social skills. We are happy with the agency we go to, but we will probably try to switch to the school to allow better communication between the teacher and the therapist. Also, it would be nice not to have to get up early on a Saturday, and right now that's the only time we can take him to Social Skills.
Finally, make sure you talk to his doctor about he meds. I would take really good notes for a couple of weeks and then talk to him/her. There are many different meds available. I have heard many families talk about how difficult it was trying different meds, but when they found the right one!!! Wow!!! It was totally worth it!!! Just make sure that this really is the right med for your son. If there is something that would be better for him, don't you want to know that?