Kidney disease is very common in cats and when it is first diagnosed, I know how much worry and torment is administered to you. But do not worry, because it is easily managed, through the diet and just general routine, do not give up on him yet! Along with his diet, make sure he has plenty of water available at all times, as he's likely to drink more which helps flush out his kidneys. Cats can live with kidney disease for many years, my RB cat Brando lived with it for 8 years, living a perfectly happy life until his last month. ((hugs)) to you and Cosmo and I hope everything works out for the best.