My Chi doesn't drink water?

Anyone else have this issue? Three years ago my nine year old male chihuahua just stopped drinking water on his own. The only way he will drink it is if I mix his food into it. I've been to many veterinarians about this and none of them seemed to know why. There have been many exams on his organs and blood content with no issues identified. I've tried distilled, spring, reverse osmosis, and tap water. I've also tried adding broths like bone, chicken, beef, and turkey to all of the different types of water. I've tried different types and shapes of bowls, a fountain, and splash pad type dispenser. In short, I've tried everything! I do frequent checks for dehydration and in the hotter months I break up his meals to four small ones throughout the day to ensure he is getting water semi frequently.

Has anyone else encountered this with their Chi? How did you overcome it?