Hi. I'm Dr. Gotthelf. I have been a vet for 35 years and I would like to use my experience to help you with your pet's medical problem.
With the symptoms of weight loss coupled with drinking and peeing abnormally, I would recommend that you have the blood checked to see if there is a disease process involved here. Chronic kidney disease, diabetes, Cushing's disease, diabetes insipidus, and others can all cause what you are describing.
Many of these diseases will diminish appetite so even if you offer very palatable food, he will refuse it.
Thank you Dr. Gotthelf,
the dog went to the vet this morning and had his blood checked. We can rule out diabetes and will get the results from, the lab next Monday. We are expecting to hear that there is, in fact, a disease. Your diagnosis was spot on.