While I hate the companies that make the foods, when it comes to things like renal failure, it is hard to do a diet on your own that is as precise.
In the old days we were told high protein could cause kidney failure. They now have found that very high is bad, but high is okay.
Once in kidney failure, lower protein is critical... so it's important that your dog get good quality.
Tufts university gives a good synopsis
For a bit more technical on why protein reduction is necessary:
There are nutrition experts who can, with your vet's input, help you to design a homemade diet. Just be careful. With your dog's dx, the 2 things that can help him feel better and live longer are diet and sub-q fluids.
We lost Arwen at 12 1/2. She was great, then early kidney failure, suddenly total failure less than 4 months after initial dx. However she had autoimmune problems and had been on steroids nearly 8 years.