could be a food sensativity, alo did you tke a fecal sample to the vet??
you need to do that to rule out parasites.
i had similar issues with my guy and the vet determined he was sensative to chicken.
"I used to look at [my dog] Smokey and think, 'If you were a little smarter you could tell me what you were thinking,' and he'd look at me like he was saying, 'If you were a little smarter, I wouldn't have to.'" - Fred Jungclaus