Is the food the expensive Royal Canin satiety one from your vet???!
That would certainly make him thirsty if you read the ingredients. Cellulose, chicken meal, wheat gluten , corn gluten, chicken,beet pulp etc. plenty for allergies . This a a diet food for obese healthy dogs!
I would go for a basic sensitive gluten free food, grain free food. Not the Pets at Home ones because they are loaded with salt. Fish, pork or lamb with either veg or potato. Or raw from your pet shops freezer , it comes as minced meats with tiny bits of bone in, and add your own cooked carrot, potato or sweet potato.
Lastly I would definitely get a second opinion elsewhere. Monday!