The vet was very helpful and offered a bunch of options if we want to try and keep him going for awhile longer, including teaching me to give sub-cu fluids at home, but when it comes down to it, I just don't think it's right to keep an animal forcibly alive when his quality of life has so obviously deteriorated so badly. If I thought he'd come around, that'd be one thing, but he's pretty obviously on his way out, and I'd rather he go with dignity.
So. I'll make an appointment tomorrow to bring him in in the next day or so and have him put to sleep.
I am, of course, terribly choked up about it and keep randomly tearing up and sobbing (my back still being out, this usually results in a sob-hunch-choke combo that finally ends with me laughing because really, what else can go wrong?) but as James and I discussed--he's had a very good life for a cat, he's been pampered and coddled and loved, we did our best with the diabetes, and generally I think he's been a happy cat. And for a stray kitten that got picked up in an alley, having been stoned half to death by small children, he got pretty lucky in life, and I guess that's the best one can really hope for.
As the song says, nobody gets off this world alive anyhow.