Well, from my perspective, it looks like your boyfriend is bottling something up inside.
You see, at the age of 25, you haven't settled in to life just yet. You don't have a wife, a family, probably a stable and profitable source of income..etc... This means that his thoughts are jumping all over the place. If he goes to university/college, he has yet another thing to keep in mind. It is simply too much to take in at such a young age. He is worrying about too many things (marriage, family, job, house...)
Or maybe he is heartbroken from a personal experience. I've had that before. If someone close to him as passed away, I can literally feel the pain. If he has just lost his job, I understand how he is feeling.
But that's not the point. The point is, that you should leave him some quiet time, to take it all in. Over the course of a week, maybe. Then, you should advise him to go see a psychologist or a psychiatrist or GP, because they will know how to help him. These professionals know what to do, because this isn't the first case of possible depression they've faced.