It was as serious as we thought. The vet said his kidneys had shut down, and said we didn't need to do any tests: that the best thing to do would be to put him to sleep.
We said goodbye and called my dad so he could say goodbye on the phone. We brought him back in a little box so my dad could say goodbye for real and bury him.
We buried him at the foot of the red maple and marked his grave with a stone.
RIP, Jonathan Harker Cat.
"He just wanted to go outside so he could come inside so he could go out so he could come in again. That is all he ever wanted in life." --My daddy.