that could be the main reason - studying overseas. perhaps he was hoping that he is staying at his own place, mixing with his usual group of friends, or maybe he was hoping for more care and love from his parents at that particular time. you see, some parent thought that by sending their children to the "best school" in whichever country is good for their children's future. but what their childrens want is some care and love from their parents, want their parent to stay by their side all the time.

just guessing though.

depression is a mental disorder that require the root cause to start "medication". without root cause, it's very hard.

probably can talk to his parents more. join more friends, go sports, take up some hobbies.