Having a serious medical issue like renal failure is a huge stress on a cat's entire system, so it's perfectly normal that he needs to sleep right now. Is he eating/drinking/eliminating okay?
As for what might have caused it, is he an indoor kitty or does he also go outside? Could he have eaten a toxic plant, or gotten into some kind of chemical (antifreeze comes to mind)? I think it would be hard to say what kind of long-term damage might have been done to his kidneys, but it's definitely something to keep an eye on.
that he makes a full recovery.