It sounds like anxiety is probably the true issue. Introversion/Extroversion has to do with how you energize and is mostly independent of anxiety/apprehension. Here is an article that might help you better identify what the true issue is. http://giftedkids.about.com/od/glossary/g/introvert.htm
I make this point because I am a highly functional introvert but I had a ton of anxiety that made it hard to function before I got help. As an extrovert, it is important for you to understand him. Does he want to change or does he want to change because you want him to change? If he doesn't see this as a problem, and you do, it will be tough for you to do anything other than accept your differences.
If it is anxiety, what is tried and true is facing the situation and doing it over and over and over again until he gets used to the fear. For me, I had a short period of social anxiety. Because I am also an introvert, it takes up a lot of energy to talk to people randomly to begin with, so coupling that with anxiety was tough. So I took a job in sales that forced me to speak with people, over and over again in many different situations so that my behavior evolved and I was more in control.
But you have to want to behave differently. It doesn't change the nature of being an introvert, but it sure diminished the anxiety and made a huge impact in my life. I would still feel some of the anxiety but I understood it and could perform at a high level for lengthy periods of time. But once I was done with the situation, I was drained and would skip happy hour to come home, relax with a beer and flip on the game.