Did he do preschool? Putting him in kindergarten is the best thing because then you will KNOW. If the teachers who are working with a number of kids see that your son is not doing what is typical, they will speak to you about it. Then you can start the process as to why. Have you brought up your concerns to his doctor? Social skills come naturally for a lot of kids but not all. Sometimes that is just due to the child's unique personality but sometimes it is due to something going on like a developmental issue. Have you worked on his social skills? Will he pretend play like drive a train on a track making train noises? pretend to be a dog crawling around the floor? Things like that? Does he make eye contact?