Water intake is certainly a cause but it can also be a vitamin deficiency. See if your vet can check for vitamins or mineral being low in his blood.
This happens to animals and people who retain a lot of water. Try increasing his fluid intake. Mix a little unflavored pedialyte in some water and give it to him for 4 days. Do you feed him only dry? Cats get most of their liquids from their diet so it is important to at least do 50% canned wet food.
Never heard of that. Very interesting. Thank you.
He might have a trick knee. Luxating patella is pretty common in cats. Especially since he seems able to pop it back into place, it does sound like that.