Vet should run an "IDEXX Canine Diarrhea Panel" from your dogs stool sample if not already been done. One of my dogs tested positive for Cryptosporidium bacteria. the panel covers other things also.
IMO, your vet has no common sense to argue against trying a dietary change.
it doesn't matter if the dogs' diet is what "you prefer". what matters is for things to be functioning in a healthy way.
first step with diarrhea is minimum fasting from food for 24 hrs minimum &/or until diarrhea stops.
irritated bowels need REST to recover from simple issues. That is basic fact.
begin solid food by trying small portions white rice. cooking rice in non-fatty, unsalted chicken broth is ok. no artificial ingredients.
If diarrhea does Not return then try increasing portions. then slowly add in some boiled chicken or turkey & gradually reduce rice, increasing the meat. Everything GRADUAL is important.
alternately, if you can't/won't try chicken, gradually add some canned dog food instead.
At least try a different brand with less rich meat as first method to wean off the rice.
If rice does not work after very careful & gradual increments, then dog may have it's own problem. sensitivities, irritable/inflammatory bowel syndrome etc. or any number of more difficult things to dx/ treat.
PS, even IF the vet treated with the antibiotic (Tylosin) for Cryptosporidium bacteria, some dogs are not able to go entirely off the med without some level of the illness resuming.
My dog has to be maintained on a minuscule daily dose. not the initial full dose.
Good luck.
