Help! Dog not drinking; has diarrhea.

My friend who I dog-sit for has a ten-year-old boxer who started having diarrhea a couple of weeks ago and then stopped drinking water. She has spent a fortune in vet bills, at two different vets, with no diagnosis. He's had x-rays and blood tests with normal results except for slightly raised potassium (by .1). He was given iv fluids once and subcutaneous fluid once, and his owner has forced water in him with a turkey baster He had a steroid shot, and started drinking again, but only for a day or so. He is on 500 mg of metronidizole twice a day. The vet said to put him on a boiled chicken and rice diet, which he eats 3-4 times a day (he loves it!) with lots of extra broth to help with hydration. The diarrhea had been almost pure liquid, then mush with blood in it, and yesterday it was soft, but formed.
The vets are thinking maybe addison's disease, or atypical cushings, but blood tests for these are very expensive, and our research on those diseases doesn't bring up diarrhea or not drinking. There is no hair loss, no thin skin, no lethargy. He is still playful, is fine on long walks and runs outside to bark at passersby.

Does anyone have any experience with such a problem in their dogs? We are desperate for answers! Thanks!