B.R.
This is actually really normal for a child this age, and is a good sign that he has a healthy attachment to you- I know it sounds weird, but I'll explain as briefly as I can. During the first year of life the most important thing a baby does emotionally is develop attachments with their primary caregivers. When this theory was developing researchers found a way to test a child's attachment- they use the "strange situation." They measure a child's reaction to a stranger with mom in the room, when mom leaves the room, and when mom reenters the room. A normal, well adjusted child at this age moves towards mom when the stranger enters, crys when mom leaves, and hugs mom when she comes back. Of course, personality also comes into play and affects the level of the reaction the child has. But unless your child absolutely clings to you and panics, he's doing just what he should. And your description of him "moleing up" sounds just like my daughter. She talks non-stop at home and around adults she is comfortable with, but clams up around anyone new. Furthermore, at this age kids don't actually interact with each other- they parallel play at best, which just means that they play in the same vicinity, occasionally observe each other, and occasionally interact verbally.
The best way to help him develop socially is to continue getting together with the same group of kids as regularly and as frequently as you can- at least weekly. You could also look for classes or activities that have her around other kids regularly- we actually just got back from a "mommy and me" class where they do a craft and sing some songs and play a game. While I was kind of annoyed by the class, it had social value. My daughter made a friend because I babysit another little girl her age, but I had watched her twice a week for over a month before the two started to interact with each other at all. Now she's good with this little girl, but still doesn't interact much with other kids. Odds are good that your son will be shy and selective about his friends, but there's nothing to worry about.