experiencing is what our Smokey (17 year old Siamese; he was 16 when diagnosed.) went through. He was diagnosed with kidney disease (although we were never told what stage). We went almost a year on the specialized diet the vet recommended even though it didn't seem to help much.
We didn't see much difference in his food intake or his litter habits. so we continued with the food the vet ordered.
February of 2020, I saw Smokey coughing/sneezing up blood. And he had lost weight in the last few months. I told hubby that it was time and we let our kids know so that if they wanted to come over and say goodbye, they had a few days (omg, i'm still tearing up typing this). We put Smokey down in late February of 2020.
Vicky, it is not an easy thing to do. But from what you are saying, Leo isn't having much of a life, let alone a good life the last week or so. You need to let him go. I'm so sorry.