[Acroyali]

The only experience I ever had with tan colored stools was with a dog who had a taxed liver from a battery of antibiotics and medications for an unrelated health problem. His liver was not diseased nor damaged, but had been working hard. This dog didn’t have diarrhea, though, so that’s something else to keep in mind.
Has he had a good old fashioned blood panel, and have his liver enzymes been checked?
Best of luck.