Monostat cream on the bum. See if that clears it up. If you CD, wash on very very hot water, like do a strip cycle or two to kill the yeastie beasties, toss in a tbps of bleach and/or tea tree oil as well...because those little buggers like to stick around.
Check to see if the inside of his mouth is Thrushy...if it is, then you'll have to treat yourself for it too.
Thrush on the bum = Possibly thrush on the mouth. Yeast = Thrush