Try to talk with your brother. Ask him how things are going for him at school. There's a chance that he gets bullied or maybe he has trouble with learning certain things. School can be heavy and when someone acts like that, there's a reason. Try to have a casual conversation with him and ask him about school. If he has trouble with learning certain things, help him out a bit. If he gets bullied, then the school counselor should know so that it can be stopped. It's understandable that you're worried about your brother. Don't think the worst just yet. This doesn't have to mean that he'd be developing a depression. There could be an easy explanation for it.