How do you know when to put an animal down?

So my soul kitty is 18 years old. He has kidney disease (very mild, labs are stable) and he has lost a lot of weight. He was 10 pounds and is now 6.5ish pounds. He has absolutely god awful smelling diarrhea multiple times a day. I have taken him to the vet, they were unable to find a cause of the diarrhea. They brought up doing an intestinal biopsy but for the reasons listed above, that doesn’t feel like a good idea. We tried oral steroids, they made absolutely no difference. He doesn’t seem to be in pain. He still eats like a freaking pig, cleans himself, and wants to snuggle. Even though he has diarrhea he isn’t having accidents.

My question is, when do you know when it’s time to put them down? Is it time to put him down now?