Most Helpful Guy
There must be a lot of people just like your guy. It can be both guys and girls. They sometimes feel unhappy, but have no real idea why. Perhaps your guy is one of these
Some people have something hidden deep in their minds that cause this, but over the years they have chosen to repress it, until the cause disappears, but the effect remains.
From your point of view, you must feel so helpless because you have no idea what's on his mind.
I think the only thing you can do is to support him, but don't keep asking about it. This could make him resentful. Just be there for him, and in time, it may come out and then you both can start to put it right.
Most Helpful Girl
Did he say he was unhappy with you or unhappy with life and himself? that makes all the difference. A relationship is two people. So, unless he said you specifically, I wouldn't worry too much. Just speak to him about it and ask him what's going on.