I don't know much about kidney problems in cats, but I do know there are diets for it - several kinds. Your vet may have them all. If Cosmo don't like one of them, he may still like the others. Can you get a small sample of each of them from your vet?

Years ago, Fister had to have Hills S/D for UTI. He didn't like the canned version, but the dry food went down just fine.