Here's what I would do:
Feed him boiled chicken for a few days, until the diarrhea clears up, then gradually start adding dry food. I would definitely give him a better quality food, like Nutro, Wellness, or Felidae. If you can get Felidae in your area, that would be best.
What tests did your vet do? The excessive drinking in a 15 yr old cat makes me wonder if he has kidney malfunction or diabetes..?
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