So now we have to feed him prescription cat food. After a month on the (quite expensive) food, they'll test him again, and if it's doing the trick, we're set. If not, we have to go to supplementing the food with shots.
The vet told me on the phone that, on the bright side, "You can tell people you put your pet on the Catkins Diet." I responded automatically with an, "I hate you, Larry." This left the vet confused. :)