So, Mom took the cat in the other day to get checked, and while things were a bit up in the air, I'm thinking they're going to get much more concrete very quickly.
The vet said there were several possibilities, which could be ruled in or out via bloodwork, but the most likely one is probably diabetes. That much weight loss in a cat of his age is concerning, and he was wondering about the whole drinking/litter situation.
The cat drinks out of the overflow cup in Mom and Dad's water cooler sometimes, but more often than not he uses his paw to drink out of the spigot, so there's really no way to figure out his fluid intake. Mom sat down and thought about it, though, and she's cleaning his litterbox much more often, and going through tonnes of litter lately. Sometimes she goes down and the whole box is wet/clumped.
I know diabetes is manageable in normal cats, but this cat won't put up with needles. The exam the other day resulted in both Mom and the vet getting injured, I believe.
Mom's going to do the bloodwork to see if it's not some other, more easily treated condition, and if it does come back as diabetes, we're probably going to try to manage it with medication. The only good thing is that the cat is acting like his normal self - absolutely no pain or fatigue or any other symptoms to indicate that he's not feeling up to par.
I wish it was something else. I hope he's going to be ok.