You know that Filou is in renal failure as well. My vet says that if they lose weight they digest their own protein which is just as bad as protein in food.
She allowed him to eat anything to stop him lose weight. Filou now survives the diagnosis for 4 years with his diet of renal dry food (Royal Canin) plus wet high fat food.
You may give him an egg yolk every other day- it's full of fat and no protein.