What you have described with your son sounds like it may all be related. In general, doctors like to give signs and symptoms a single cause. However, you have added headaches into the mix along with concern for disasters like a brain tumor. I don't share your concerns about a brain tumor at this time. A complete physical exam by his doctor along with a more detailed history may help put your minds at ease.
Have you tried talking with him to help figure out what he thinks is going on? Adolescence can be a difficult time for both kids and their parents. Teens often have many things on their minds, accounting for a lack of activity and motivation. Overall, I would "quit worrying" at this point. I would still keep an open mind for further evaluation if, in fact, you do notice a continued significant change in his school performance, his personality, or if any symptoms persist longer than you're comfortable with.