My dog ate cat litter three weeks ago and got diarrhea. We took him to the vet and he was prescribed Metronidazole which helped a bit, but now that he is done his dosage, his poo is mucus-y and abnormal looking again. It has been very soft and pudding-like for three weeks now. The vet also took a sample of his feces and the test rescults came back negative. He eats, drinks and plays normally. What can we do to make him better?