You can administer fluids subcutaneously. Plain water is fine - inject some with a regular syringe under the skin. Just inject S-L-O-W-L-Y. A tiny amount at a time. It will absorb into the tissues, keeping him hydrated.
(If he needs fluids more than a few days, you might want balanced, electrolyte ones - but for one or two days plain water works well enough.)
We have a vet appointment in an hour but have set a limit and are prepared for the worst.
I am cautious about fluids since the last big crisis where Oliver used up a couple of his lives (at least, and one of mine!) was a blockage and fluids were contraindicated until his bladder was relieved.....Hence the 1cc at a time, and only 3 times.
Kind of sounds like what happened to our cat a few years back. But after the eyes being dialated. He started to go into seizures. We took him to the vet. And the vet said his temperature was way to high. So the vet gave him some meds, and now he is alright. I didnt have money for extensive testing. But our vet is a curious one. He did some of the tests himself. He said that he was showing signs of some kind of poisoning. Like weed killer, antifreeze, or rat poison. But when they did blood tests, no poison showed up. So he was at the end of getting the poison out of his system.
The high fever caused the cat to become badly dehydrated. Which caused the dilated pupils and the seizures. After giving meds to make the fever go down and 4 days of fluids through IV, our cat is fine.
I had to put him down. It was neurological (no surprises here) and was either a stroke or brain tumor. He said rabies at first (I wasn't sure if he was up to date because the vet we used last time he saw one went AWOL, closed her office without notice suddenly) since it presents the same way in cats. But I asked if it would come on so suddenly, as he jumped onto a hutch for his breakfast out of reach of the dogs and literally was yowling, flipping, eyes dilated 3-4 hours later. Nope, he said, not rabies. Whew.
It is a sad day here but honestly, we did a lot of crying since yesterday because we strongly suspected this would be the case so we took the time to say goodbye.
I stayed with him and held him. He went before the plunger was pushed all the way. The vet was surprised, said he went much, much more quickly than usual. I think he was ready.