Does he smoke? I think that decreases one's libido? Or drink? Is he stressed over anything? Heaps of things can effect your sex drive, despite us thinking men are always up for it (well, 90% probably are).Or, and you don't have to answer this and I hope it doesn't offend you, does he masturbate? That would surely affect the desire to BD?
Ach, I don't know. Good luck!