When your son has children come to your home, use the time to observe the way he interacts with other children and how they respond to him. This can be helpful in teaching him the skills he needs to fit in with others. For example, if he is bossy, you can remind him that it makes other kids feel uncomfortable.
At school, it would be worthwhile to speak with his classroom teacher. Tell her your concerns so she can monitor his and other children’s behavior on the playground and intervene when necessary.
You might also want to question what kinds of accommodations the school is making to assure that his work is challenging and interesting to him. If you think that he has some obsessive-compulsive traits, you may want to speak to the school psychologist or guidance counselor to see whether this is interfering with his social skills or academic achievement.