I'd also suggest having your vet show you how to palpate your cat's bladder. It is possible to check it yourself to see if it is full. But there are other issues that dehydration can cause short of a UTI or obstruction, constipation being one of them and, obviously, chronic dehydration can lead to kidney failure. Finally, I'd note that you should determine, if you haven't already, if your cat has any crystals in his urine and, if so, their type. This could necessitate other changes to his diet.