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I am Dr. Snow, and I can try and help you. Did your veterinarian run his stool sample to check for parasites? If not, see if you can drop one off to make sure he doesn't need a dewormer.
I would suggest adding in some additional probiotics, such as Align, or cultured plain yogurt on the food.
You can try a bland diet for a few days to see if that resolves the diarrhea. I would do boiled chicken and rice, mixed with a little cottage cheese.
If the diarrhea continues, it may be a sensitivity to his food, and you may consider changing to a limited ingredient grain free diet.
Let me know if I can help further!