Without knowing your son individually, here are a few possibilities:

A.) He is being bullied by peers

B.) He could be stressed by his body image (compared to what he feels he must live up to)

C.) He might be suffering from self-esteem issues, especially if measuring himself against the skills of classmates

If you're still close with him, he may benefit from conversations about some of these topics. Or, he might need another male person to confide in about what he's going through (if that's the case, don't take it personally -- it isn't about you, and it doesn't mean he loves you any less).