Thanks for info on foods Growler.
We had ultrasound not Xray, and there's nobody around here who can do the surgery, closest is a day's drive which the cat won't enjoy, and given his age it's not certain they'd operate. Our vet doesn't think he'd survive the surgery. Even a biopsy would be invasive and can't be done around here.
So we're not sure his kidney has failed, but the effect is the same. He had bloodwork done, so we know his toxicity is increasing.
So far he remains active, interested and normal, just yellow.