If your cat is doing better, then great. But consider what is in his best interest, rather than yours when doing everyting to prolong his life. It hurts to lose a pet, but at 17, and with kidney failure I suspect he is not long for the world no matter what. If it is something potentially reversible, great, try what you can, but if he is suffering let him go. Think about the quality of life that he is enjoying, not the sadness you might feel if you lose him.
Last year I had to put down my 16 year old dog. He had a great life up till the last few months, but when he started losing weight, not eating (which he LOVED to do) not wanting to go out, and generally just languishing around, I knew it was time to say goodbye. One of the hardest things ive done, but also one of the best things i believe. Im sure he would have kept going for months, with meds, diet supplements, frequent vet visits (which scared him to death) etc, but the best thing was to help him go peacefully. He died in my arms calmly, and in no pain.
