I just happened to find this post through a google search of how to overcome shyness. Let me first say that I'm not a mother. In fact I'm not even female lol, I'm a 20 year old college "kid" with a shyness/social problem. You all know secondhand the most about your children, with only them having more knowledge than you. So I'm not telling you that there's nothing you can do, I'm only saying that your generation and our generation are somewhat different. I hate to say it, but as far as high school goes, he doesn't sound like he has much chance of a fresh start. High school is just like first grade in that once the kids get accustomed to the newenvironment, the split off into their own cliques. And once that happens, it's a musical chairs effect where those left standing (without a clique) are just left out. I know it probably hurts as a parent to hear that, but it's the harsh reality. Unless he can completely rip out of his shell and do something very memorable to his classmates, college will be the chance for a fresh start. This is all assuming that he is not, in fact, an introvert. If he is, then he won't change because he feels no need to...he's happy the way he is. But like I said, you are the one to make that judgement.
Also, just to be safe, check to see if there aren't any mental illnesses that he may be suffering from like depression or social anxiety. If so, then it's going to take a lot more than a change of style to help. It's a serious matter that is soooo unrecognized. Not to give a pity story, but I personally struggle with depression/low self esteem and the occasional suicidal thought.
So basically if you want to do some good, start prepping him for college life by encouraging him to stick with a few friends for security. If he has none, work with him on learning how to break into the social circles. I hope this post helped and I apologize for any typos, I typed this on an iPhone haha