shy and introverted adolescent

What should I do if I fear my teenage son may be too withdrawn? When he was younger we thought it was just a phase and assumed he would gain more confidence as he matured and interacted more with his peers. So far those expectations haven't panned out. He doesn't seem to have any interest in developing relationships with friends his own age (or with anyone else in the family, for that matter). He spends most of his time alone with the television, his computer, or video games. What are we up against and how should we deal with it?