Well, I don't understand why the vet didn't give your cat prednisolone instead. But, regardless increased drinking and urination are common to both - not the reverse.
The only other thing to do - IN ADDITION to calling the vet - is to look around to see if he has peed somewhere else besides the litter box. I noticed my cat was not using the litter box as frequently as normal, and it turned out she was peeing other places. Given her CKD, it was hard to see or smell because most cats with CKD have dilute urine. She is being tested for a UTI as we speak. In the past when she has had UTIs she never cried or peed outside the litter box, so those symptoms cannot be relied upon.
The only other thing to do - IN ADDITION to calling the vet - is to look around to see if he has peed somewhere else besides the litter box. I noticed my cat was not using the litter box as frequently as normal, and it turned out she was peeing other places. Given her CKD, it was hard to see or smell because most cats with CKD have dilute urine. She is being tested for a UTI as we speak. In the past when she has had UTIs she never cried or peed outside the litter box, so those symptoms cannot be relied upon.