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.
He had a very good life.