I would take him to the doctor and ask to get him refered to a peadiatrician. He's probably just introvert, but it won't do him any harm to get checked over. Doctors don't like to rush to diagnose these days so they will usually (depending on where you live!) be very thorough and go over every eventuality. My daughter was a bit like this, but she had hearing and speach and language problems which have since been successfully treated, and she is a relatively 'normal' child now.