I used to be a vet tech, so I've seen instances like this many times. Several factors can lead to diarrhea: 1) Parasites, 2)viral or bacterial infection, 3)change in diet, 4)stress, 5)chronic medical condition like ulcerative colitis. All these are good possibilities, among other things. If your vet has ruled out parasites and the dog has been treated for infections, then I would attempt to stabilize the diet with something bland. Many dogs have very sensitive digestive systems. Any slight changes can throw everything out of whack, commonly leading to diarrhea. Another respondee wisely suggested boiled chicken with plain white rice (no seasonings whatsoever). If you haven't the time to cook, some vets carry foods like Eukanuba's Low Residue or Hill's i/d. Both are good, bland prescription diets meant for easy digestion and can only be bought/prescribed by vets. If it is in fact stress, try your best to eliminate the factors that have abruptly changed in your dog's life recently. Bring it back in the house and confine to an easily cleaned area - tile floor, etc. and make him comfortable with some cozy old towels (something easily washed or tossed). Otherwise, it is a good idea to check back in with your vet and get his/her suggestions. If your vet will not take you or your dog's problems seriously; it is time to find a new vet who does.