How long has he been like this Julie? is this a new thing or is his nature generally quiet? What are his friends like and how is he in their company? Not sure what the solution is, but I know how you feel, you just want everything to be right for them don't you?. We are awaiting a referral to the school pyschologist for my 10 year old as his self esteem and confidence are really low - he does have a quiet nature, but was always happy in his own skin if you get my drift and was happy enough to speak to familiar adults etc. A couple of years of bullying have affected him though and he seems to have lost his ability to speak to anyone outwith his immediate comfort zone and now doesn't even speak to people that he would have before. He stuggles to make eye contact.
Just wondered if maybe your son was having any other issues that he is keeping from you.
From things I have read, it is important to ensure that he has independance and control over things where possible in order to increase his self confidence. Praise as much as possible as well. What about a bit of a makeover?. Does he fancy a new haircut, some new clothes etc. we all feel more confident when we feel good about ourselves. What about something like an adventure course, like sailing or Duke of Edinburgh or something like that which would give him a sense of achievement and pride?
It will probably be a few months before we get any help from the psychologist, but I will happily pass on any ideas we get.
Hope things get better for him.