Talked to the vet twice today. Nothing particularly good. The mass indeed is his right kidney; she thinks he may always have had a smaller left kidney, and it's given out, and the right has been trying desperately to meet demand. But his intestines show changes as well that may indicate lymphoma. And the heart murmur for which he was checked years ago is reappearing, slowing down the process of rehydrating him in order not to overload his heart. On the plus side, his creatinine levels were dropping as they did get him fluids. She wanted to keep him another night but was optimistic that he can come home tomorrow.
Went to visit him during visiting hours... oh God. I suppose in retrospect that he's sedated (otherwise he'd have chewed up his IV tubes, trust me) but he looked hellish, so very much worse than when I brought him in. He was aware enough to know me and purred continually while I rubbed his head; I was fortunately only overcome once, given that he had the penthouse cage and his downstairs neighbor's people were comforting their critter too (and it was a bit challenging not to, y'know, cry on them).
In the computer chair where I'm sitting right now.
In the interest of fairness, his brother (with the laundry).