Sudden personality change is often a sign of some sort of psychological disorder, but with the symptoms you gave, it's hard—almost impossible, in fact—to give a real diagnosis. I've studied psychology in some detail, and I have three very good friends who major in psychology (one currently holds a degree in psychology, in fact); I also have two good friends who both suffer from manic depression, but it's very hard to give any diagnosis with what you gave.
You say his mood changes, but how fast? The mood changes associated with bipolarism don't happen on a daily basis, for example.
I wouldn't jump to the conclusion of depression. A lot of teens suffer from bouts of sadness. School is hard, social lives can be stressful, and on top of that, teens are going through major hormonal changes that can affect their moods. It may not be depression—it may just be adolescence.

But again, it's hard to say. I don't think we have any psychologists or psychiatrists on cB, so it'll probably be hard for anyone to give a really accurate answer to your question.