I'm in agreement with what Mary and others have said in their posts. Fluids, fluids, fluids! My RB cat Jade was diagnosed with kidney failure and it was found during blood testing for an upcoming dental as well! My vet gave her a poor prognosis. She was 16 at the time and I was devastated. At first my vet wanted her on Benazapril chews for her organs and fluids done by the techs and low protein diet.
BUT... she wouldn't eat the special diet and was losing weight so my vet said just feed her whatever she likes. Fancy Feast and Friskies were her favourites and she ate that with gusto! I stopped the meds because they were very expensive and I simply couldn't afford them at the time. (Jade didn't have a heart murmer though like Ash and my vet wasn't entirely sure the meds would help her condition anyway).
I also started doing the fluids at home and it's really affordable this way and much less stressful on the cat. Jade went on to live to 21 years old. Don't give up hope!!
p.s. I know you said you're not sure about giving him fluids yourself but if you see that he is getting dehydrated (has that separated fur look and can be more lethargic) you can ask your clinic to set up a time with the tech and they can do a treatment in 10 minutes or so and show you how. You could watch them do it for a few weeks and then decide if you want to give it a try or not. You can get a smaller gauged needle that is very fine and it goes in very smoothly. I used to give it to Jade just between her shoulder blades. Once you've put the needle in, that's the worst part. After that the fluids drip in for 5 or 10 minutes and then the cat tends to feel so good afterwards and have much more energy.