Ahhh I really feel for him. I loved languages but was too lazy (rather than shy) to do a school exchange - didn't do me any harm and I still ended up with a good degree in two modern languages, so I have to say I don't think it's the be-all and end-all in terms of language learning. Having said that, it can be enormously valuable for some kids, particularly those who struggle to "contextualise" their language learning, IFKWIM. There was an interesting article in the Guardian a year or so back about a mother who basically packed her kid off to France for a few months (or maybe it was even longer) and he came back pretty much fluent, and much more confident/sure of himself (but perhaps he wasn't so shy to begin with).
This is slightly off-topic, but when I was growing up I worked in a local village pub and restaurant, as a waitress and barmaid, and I have to say it was probably the single most important factor in me gaining confidence and the ability to chat to/get on with all sorts of people. I was fairly confident to begin with but I saw it to an even more extreme extent with other girls I worked with, who started off unable to say boo to a goose and ended up sooo much more socially confident. Is this, or anything similar, an option for your son?
And finally, what about considering asking him whether he'd like to go on one of those PGL type trips abroad, where it's perhaps not so scary as he's just in another family's home, with one other (potentially sullen) teenager, but he's basically with plenty of other kids his own age, most of whom would probably be English, but at least he'd be spending time in a foreign country and making steps in the right direction?