Don't stress about him at all.... My eldest was exactly like that. Even as a toddler he still didn't like strangers coming up to him. I used to think it was hilarious that a stranger would come up and try to engage him in a conversation then say something stupid like, oh doesn't he talk. It was more a case of actually he just hates people who he doesn't know shoving their face in his personal space! He was a lovely chatty and friendly little boy with people he knew and trusted but with anyone else, he would just look at me as if to say "WTF??"