Cushings comes to mind. Excessive thirst is one of the most visible signs along with a longer hair coat that doesn't shed out as quickly as others. A blood test can confirm. Is he peeing more also? Is he eating more protein? As in Alfalfa or a rich cutting of grass? Higher protein needs to washed out of the kidneys and they drink extra water to take care of it. There are a few vets on this forum, they will probably come along and explain better.