Is he on anti-depressants? One of the guys I've been with is on Zoloft, and it turns him into a marathon man...it also, unfortunately for me, makes him uninterested in sex for days at a time. If he's not, I'd say he should probably see a urologist to rule out any health problems. Other than that, it could be a mental issue, and that's something that you and he will need to figure out together.
Good luck.