In my research years ago with a senior with KF high protein is not detrimental to the kidneys. It is the "quality" of the protein (dog food) that is important. Low protein will not benefit a dog with KF - and muscle mass can be lost.
Wanted to address your question on 20% fat for a senior less active dog - I agree it would be too much and if he gains weight from it, it will be detrimental to arthritis.
I like at least 28% protein for a senior (or any age Lab) to maintain muscle mass and there are a number of foods on the market at that percentage and higher that have less than 20% fat.
I am feeding a 28/17 food. My senior girl, almost 12, does well on it and is not over weight. She is not real active either. I also have an 8 year old and 2 year old on it and they are a good weight as well.
I feed this to my seniors: http://www.holisticselect.com/senior-dry-dog-food-chicken-meal-and-rice#guaranteed-analysis
Because it is 10% fat. I also add some additional protein. Usually enough to bring it to about 28%. They formulated this product a number of years ago. I have never fed a senior food until they came up with this. They used to be all crap. They finally got the right idea on protein and old dogs.
The fat is the problem with my guys. I need to keep the protein up, but with 16% fat plus, they gain too much weight over 9 years of age.
My 13 year old senior has been doing great on Pinnacle Salmon and Potato 27/14. He is active for his age but has slowed down. He still maintains solid muscle mass and I have not had issues with weight.