16? I would say it's typical teenager syndrome (TTS--just kidding). Without knowing more about your son's situation (school setting, peers, etc.) it seems similar to most 16 yr olds I know--at least around grown-ups.
If we take my DS15 away for the weekend, we see the old DS we know and love. He's so pleasant and interesting and helpful. If we stay in town and he hangs out with friends and goes through normal routine, he's more solemn or moody. Part of it is certainly school work, but part of it is peer pressure--not cool to hang out with your folks, chat, joke, etc.
One note--if you suspect he's depressed or has issues, make sure you try to talk to him and see if he needs help or intervention. 16 is such a tough age.
