Thank you all for coming and pitching in!
I have a nice honey bath with dechlorinated water in his hospital tank. Already, since last night, much of the rash-looking redness has dissipated, and my little guy isn't itching his belly against the bottom of the dish. It's getting much better, so I will keep up with the soaks and go purchase some Pedialyte.
As for the "how" of it, he went on an unauthorized jaunt through the house, and gathered a nice hair-and-dust ball. Far be it from the imagination that such things are sanitary.
Keeping a close vigil for now. Thank you all again!
Kind regards, and happy winter season/holidays/Christmas/Hanukkah/weekend/etc!
Phil E.