Dear M.,
I, too, have a very shy, (I think) very bright little boy! My son is the same way; he has wonderful relationships with close family members but is terrified in group settings with other children.
I've asked similar question on Mamasource myself, and I've gotten a lot of wonderful feedback saying that shyness and introspection aren't necessarily bad -- they can be wonderful qualities that come with wonderful kinds of intelligence. However, shy kids are often very sensitive, intuitive kids, and it's important not to make them feel bad about being shy -- I don't know about your son; mine internalizes everything.
My own approach is to try to find a balance between encouraging my son to be more comfortable with others but not forcing him to override his natural caution, and not making him feel bad about being shy. My son is enrolled in a very part-time preschool (a total of 5 hours a week), and while he's shy and withdrawn in that setting, the increased exposure to other children has helped him share space better -- he can now play at the Barnes & Noble train table right next to other kids, etc. We've also just scheduled our first one-on-one playdate at our house. I'm hoping the familiar setting will make it easier for my son to interact socially.
So .. I guess my recommendation is to give your little guy small, managable opportunities to interact with others but continue to build his confidence in areas that interest him. You might also look for a kids science class, so your son can meet children with common interests, though most of them start at age 4.
Finally, I wanted to reach out to you and sugggest that we keep in touch, or perhaps even schedule a playdate ourselves, since our little boys seem to have so much in common. We live in central NJ; my son will be 3 in July.
Send me a private message if you like :)
Mira