[kjc]

Has the vet prescribed any antibiotics? Clostridium (bacteria) is very common esp. with all the wet weather lately... I'd also keep him on a bland diet for a few weeks till this clears up completely and not mess with changing foods, as that can also stress the intestines. Also ask about adding probiotics to his food.

If nothing is showing on a fecal exam at the vet's, I'd have a fecal sent to the lab for more thorough testing.