My 16 year-old has always been an outgoing, social kid who plays competitive sports and has lots of friends. For the past two months, he spends all his free time in his room, hardly contacts his friends, rarely talks to us, and has quit varsity basketball. Chalk it up to being 16.
Teens often go through moody, withdrawn periods.
There's cause for concern. Get him some professional help. These dramatic changes and withdrawn behavior have lasted too long.