If all his tests have come back NAD and you are already trying Chappie, what about adding a teaspoon of slippery elm to his food twice a day, it helps bind them up (not so they are constipated I might add, just firms up loose stools and soothes tummy troubles). I always keep some in and found it really useful for Phoebe's pups when they were being hand reared and sometimes they got a bit sloppy, so just added slippery elm into their meals and next day poops were fine. It could be worth a try?