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.
Never heard of that. Very interesting. Thank you.
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.
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.