C.M.
A successful treatment for your son will depend largely on what his specific issue is.
But what has worked for us is a lot of playacting. We always start with using toys like a Barbie doll, or for your son some kind of person toy. You would start by making up a situation (like joining a new group). Have your son be a member of the group and you be the new person. Go over different things that everyone could say. Start gentle with lots of positive reactions. Then when your son feels comfortable, have him be the "new" person toy and you be a member of the group.
Over a period of time when your son gets used to this "game" you could slowly introduce more uncomfortable situations that your son may encounter. If he's comfortable enough, you could find out what his fears are. Maybe he's not sure what to do or where to sit. Handle each fear gently by you first demonstrating with the toy what to do (and having him be a member of the group, or even the group leader) and then he gets to try it with the toy.
Don't be afraid to be a little bit silly with the game sometimes and keep it light and fun.
Once he's more comfortable with that, you can go on to straight playacting. You be the new person, he gets to be a member of the group. You demonstrate what to do. Then let him try it. Eventually he'll feel more comfortable.
You don't need to play the game for a long period of time, and you don't have to do it every day. If he isn't really in the mood, don't force it. Be sure to stop the game at a high point each time, like when he does a good job or starts laughing.
This has always worked for my daughter in resolving all her social issues, whether it be a bully, her being shy or not knowing how to handle something. It's to the point now when she's uncomfortable or doesn't know how to handle something she will bring me the Barbie dolls and say "this girl said something and I didn't know what to do, can I show you?" Then we'll act it out with Barbies.
I don't know that this is a solution for your son, but if he is just really shy it should help. If it's something more, a specialist may be needed. Good luck!