Ghilly has offered some good suggestions. I would also wonder about kidney diseaes, given the loss of weight and thirst. This could be discounted with blood tests at the vets. I assume they checked blood results for kidney failure?
In the short term, it seems it is important to get something nutritional in him. Personally, I would advise something like Green Tripe, which you can usually get in tinned or frozen form from a good pet store. If bought frozen, cook a 2 inch square chunk in the microwave at 30 per cent power for 10 minutes. It must be GREEN tripe, not white tripe used for human consumption. Dogs invariably love it, even though the smell is a bit offputting for humans. It also contains lots of nutrients and minerals, while being fairly gentle on the digestive system. I've never known a dog yet to turn its nose up at green tripe, so give it a try if you can (mix with a little cooked white rice). Tony
I'm so sorry you're going through this. There is NOTHING more frustrating than having an animal be sick and not be able to figure out what it is. If only they could talk and tell us what they are feeling.
Do you live near a veterinary college? My best advice would be to take him to a veterinary college to see what they can come up with. They have state of the art diagnostic equipment where many small vet hospitals and clinics do not.
Since you said that they have tested him for numerous things, I am assuming they have tested him for diabetes? Thyroid? Cushings? Have they done any ultrasounds or MRI to rule out a blockage? The fact that he drinks a lot of water makes me think diabetes or Cushings, but I can't imagine a vet running tests on a dog who is losing weight without testing for diabetes. I also cannot imagine a vet suggesting that you have to make a decision if there was no progress without doing everything possible to determine what this is! Vets shouldn't just throw up their hands and quit! Find another vet in that case, one who is willing to go a bit further to see what the problem is.
Sorry for my post being so disjointed. I just spent two days in the hospital myself with a horrible stomach flu so I am just typing my thoughts as they come rather than making them into a coherent paragraph. My apologies for that.