if he's that bad off, maybe taking him to the vets to be put down would be a good idea. If he's suffering but still has 3 or 4 days, or more left in him, do you really want to let him suffer longer than he has to? It's a hard thing to do, my mom had to put down quite a few cats over the years. It's just hard to see them in a state where you know if it's not over for them, it's damn close.
about 10 years ago one of my moms cats started to show signs, the cat was around 15, didn't notice anything before this day. She took her to the vets and it was done for, started having a hard tiem breathing while at the vets and they couldn't do anything for her. Oddly enough this was a pretty mean cat, but that morning the cat was super nice to me, I gave her milk and water and she was actually affectionate towards me. She knew her time was near I think, was weird to have the cat be nice to me after 15 years of hissing though ahah