Well, at least now that he is out and about socializing with the other cats, he has a quality of life rather than living in a room by himself. Kidney disease isn't a curable thing. We do what we can, making them comfy and happy, for as long as we're able. Right now that's what you are doing for him, and from what you tell us, he seems to be enjoying himself. What more could we ask for? Go Braveheart!