There is this guy who is my friend.. but i dont know what to say to him.?

I have a guy thats a friend. Hes really nice most of the time. he never smiles though just laughs. But sometimes he can get really emo or angry. Like sometimes in homeroom he will walk in sit down in his seat and just put his head down. when I asked him whats wrong he lifted his head and just stared... His eyes were red like he was crying and wouldnt talk. Later that day he seemed fine though.. But sometimes on the computer he vents to me and says he hates all his friends.. he says he wants to move because they never listen to him... I listen to him but he never tells me whats wrong with him i dont know why. someone please help me help him!