Yes, Diflucan should work if he is carrying yeast. It is also used for treating systemic candida, which all people can get. It's a nasty drug, though, and can make people feel crappy. Could he rinse his parts with boric acid solution twice a day for a week or something? Or boric acid solution mixed with a few drops of tea tree oil? Or apply diluted cider vinegar with tea tree or lavender essential oil for a couple weeks? (the last one was very effective for me on a skin yeast infection I had, and I think YIs on male parts are usually sort of surficial, so that would probably work. . . but the treatment does take a few weeks to be completely effective.)

But if he's had the yeast infection for a long time (which is likely since you have had it for 3 months), maybe taking the Diflucan AND doing the topical stuff would be a good idea.