Okay so if he's been prescribed with medication he has seen a doctor before. Is it a local doctor, or where he used to live? If it's local, try finding the doctor's name and talk to his doctor. Tell him what you're seeing in your husband and try getting him into the doctor as if he needs a "check-up". That's doing it kind of behind his back/lying a tad, but at the same time it could be really worth it. That's an option and getting him help.

Also, if he's open with you... you can try talking to him when he's in a good mood and it's an appropreate time. If he's not that open, just ask some things like "How was your day?" or just little questions like that. Or mention something that relates into his problems. Maybe saying something happened at work or somewhere else. Also, maybe say something that might get him to say something to you about his problems. But don't force him, and if worse comes to worse I would confront him about it. I don't know how your boyfriend is with anger or anything, but still it's that last thing I would have in mind to do (well, basicly if he would be angry if you mentioned he needed help or he would get even more depressed)

I don't know if I did much help, but if you need anything else I'm sure I could help you. Good luck! PM me if you need anything.