Finally this afternoon, Tara found him sitting in the litter box. But it still took us an hour or two before we decided it was looking bad. I ended up taking him to the UC Davis affiliated emergency Vet clinic, apparently just in time.
It turns out that he had a blockage in his urinary tract, but because of the potassium and the pain, he was suffering from tachycardia. However, the vet was able to give him dextrose and insulin and got the heart back to normal, and was able to catheterize him to clear the blockage. He is still in ICU, and will be there for another day or two, and this is costing a lot more than I ever imagined I'd spend on vet bills, but given our current stress and mood, especially Derrick's, having a dead cat would have been too much stress.