I have picked up the skinny kitty from the vet, who will call me tomorrow with the test results. His physical exam seems to have gone pretty much fine-- he got normal on:
Coat & Skin
Nose & Throat
Central Nervous System
He was abnormal in Mouth/Teeth/Gums, being listed as having tartar and gingivitis. This is probably not surprising. And I don't think, since nothing else under "mouth" was circled, that this is enough for him to be not eating dry food and therefore losing weight. I will ask about this tomorrow on the phone, though, to be sure.
The only other "abnormal" was Musculoskeletal, which, yeah, we knew. He's Skeletal-Cat.
The vet also wrote, "Gandolf has (I suspect an old) injury to the right hip." I'm not entirely sure what that means, but, since he is not having to hunt his food (given that we feed him), I also don't think that that is the cause of his skinniness. (Although it doesn't help his overall well-being, obviously.)
Also, unsurprisingly, she found flea dirt. (Of course she did.) I would expect nothing less from a stray/semi-feral.
Gandolf is currently still in the carrier, on my porch. The vet tech who brought him to me suggested that he not be released immediately, especially since they couldn't get him to drink any water. AZMom is, wonderfully, willing to take him in for a few hours when she gets home, so we're hanging out waiting for that. They just want him observed for a few hours before he's released, to make sure he's safe from the anesthesia. (EDIT: We're off to AZMom's now.)
So, keep those fingers crossed that whatever's wrong is treatable and that we can help Gandolf. I will report again tomorrow afternoon, once I've heard something from the vet.