Approximately 25 to 30 percent of the general population fits the description of an introverted personality. That is, they don't need a lot of external interaction to be energized. Their focus is more on their inner world of ideas, thoughts and reflections. It's normal for this to be even more clearly expressed during the teen years.
Isolating himself, on the other hand, is not healthy. Isolation may be symptomatic of depression, drug or alcohol use, anxiety or illegal behavior. Here's how to tell the difference. Symptoms of problematic isolation include secrecy or defensiveness when asked about his activities, anger, aggressiveness, declining academic performance, lethargy, difficulty concentrating, trouble with authority or pushing away existing relationships. Then it's time to talk with him and/or get help from a youth pastor, teacher or counselor.
If all other aspects of his life seem normal, it's likely that a healthy introverted personality may simply be manifesting itself. But better safe than sorry. If you're still uncertain, talk with him about your concerns.