Definitely get him on fluids if at all possible. I'm surprised that the vet hasn't recommended this already at this stage.
I've had two cats with kidney problems. One with chronic renal failure, whose lifespan was extended by my giving him daily fluids. Another with less advanced kidney problems, whose kidneys improved for quite a while after I gave her fluids first every other day, then twice a week.
Sub-Q fluids can help so much. I even gave some to my cat who recently had problems with FLUTS (feline lower urinary tract syndrome). The vet had mentioned that some feline vets suggest this course of treatment, and I was quick to volunteer to do it, because I'm pretty much a pro now at doing sub-Q stuff (after giving twice-daily insulin shots to one cat for about 4 years, and giving sub-Q fluids to three different cats (the two with kidney problems, and a few times to my diabetic guy when he was feeling punk).
It seems quite daunting before you try it, because the needle is big (it actually seems HUGE compared to the thin little insulin needles), but if you grab the ruff, where there are fewer nerve endings, massage it a bit to kind of deaden the area, then pull it into a tight tent into which you push the needle, it's pretty much painless to the cat. The biggest problem is figuring out a position that is comfortable for you and the cat for several minutes. I usually would do it on the bed, with the cat between my legs, and the fluid bag hung over the back of a door on a hanger.