It is a difficult transition for some kids. I'm a middle school teacher. Instead of one teacher he now can have as many as 8 in a day - all with different expectations. He may be starting to mature, or feel that he's not maturing as fast as his friends. There may be sex going on around him that he's uncomfortable with (or at least the talk about it). Older students may be bullying the younger students. An old grade school friend may be finding other friends besides him. He may be feeling overwhelmed by the work. Has he given you any indication about what's going on? I've done a lot of studying about the adolescent brain... it is developing faster now than any other time after infancy. There are some great books out there on the subject. Good luck!