It's impossibly to really tell just from a post on here if it's shyness or more. But IF it's more, it sounds like it could be the beginnings of selective mutism. Basically selective mutism occurs when a person acquires language normally, but then does not speak at all in certain situations. It's an anxiety based condition.

One thing you might think about doing is as a family doing some kid's yoga dvds or something like that. Learning to relax his body and control his breathing will be really helpful if he does have anxiety, but it can also be really helpful for everyone in life. So it's a step you could take that will help him if there is a problem, but also isn't jumping the gun. And since it's not directly related to him not talking, it won't add pressure. It's just something fun you can do together. My kids love the Yoga Kids series, and the Silly to Calm dvd has a lot of breathing exercises.

IDK if that helps.

DEG