J.B.
I wonder if you arranged a one on one play date with a kid (or kids) in his class in your home or a park or something, then he might feel like he has a friend there. I was a pretty shy kid and my 12 year old is quite shy in social situations, but will talk your ear off in a one on one situation.
I think to a shy kid it is looks as though everyone already knows everyone else and they are already all friends since they talk and play together. It is like the shy child somehow missed the get-to-know-you mixer and is feeling on the outs. Some out of the classroom playdates with several other kids might help him break into the social mix.
It is hard to be patient with shy kids-even being one myself. I always think "I got over it, why can't he?"
Good luck.