Is Fatty on any monthly dewormer or has s/he been checked for internal parasites lately? I would first try to rule out any internal parasites that could be causing the problem. It's relatively inexpensive to check for and treat, given the infestation isn't too big.

If he isn't on a monthly dewormed then I strongly suggest one not only for him but for all the cats because chances are if he's infested they probably are, too.

Also, just because you don't "see" any worms doesn't mean they aren't there. You can only see one type of internal parasite with the naked eye