DD1 was the same. Painfully reserved, and she still is, even though she is getting a little better. Honestly he will come out of his shell when he is ready. but theres nothing wrong in getting him into more things where he has to interact with other kids. Also try and pick some activities where you are not needed to be right next to him, perhaps where he can see you or know where you are, but you arent being right there next to him? Or do this gradually over time? I found preschool really helped DD1, but I had to wait until she was ready to go.