Have you spoken to him about it? Has he said why he isn't eating? When you sit down for a meal together, do you enforce any rules? Or at least try to so you can get some idea of why he might be doing this? I ask because, if he's developed an eating disorder, that's a psychological problem. If he's not eating because food is making him ill or something of that nature, it might be a physiological problem. At some point, you have to get him back to eating or he might suffer developmental problems. It's pretty much up to you to see what's up, or if you can't, maybe he will talk to a counselor at school or to your doctor.