A few things come to mind...

a) he could be depressed, or on his way to depression.
b) I've heard of low testostrone levels in men contributing to this..
c) he could be cheating - there are signs to look for with that, and
I'd I'm not sure of them. -
d) You're relationship is for all tense and purposes over, and its just a matter of time.

I don't know if its one of these, a combination of them or any of them at all. You obviously don't feel good about it, so you should definately bring it up with him, and if you want, go see a doctor/consellor about it.

good luck!