How much do you invite his friends to be part of his out-of-school life? For example, do you allow him to have friends to play with him in your house? (I knew a mother who wouldn't allow her son to bring friends home because she seemed to be more worried about keeping her house clean and tidy than helping him to develop relationships. Grrr!). If you take him to the beach, to the leisure centre or to the cinema, do you invite him to bring a friend along? Children often seek out friendships that they can enjoy both in school and outside of it; others might not want to play with him in school if they know that they can't extend that friendship out of school.
If you get really worried you might consider having him screened for high-functioning autism. It's generally accepted that at least 1% of the population are on the autism spectrum but many people think that the figure might be closer to 5%, so it's something to keep in mind. (I'm on the autism spectrum and I know plenty of others who have either been similarly diagnosed or who clearly show the signs of autism, so it's obviously far from rare).