They don't appear to have wet food for cats unless I am overlooking it somewhere.Have you looked into Inception pet food? We're trying it for our pickiest cat because it's one of those no grain, no soy, no wheat, no legumes, sort of foods that's meant to be as pure as possible. Not sure how it works on the phosphorus side of things, but it could be an option? You can even get a free sample: https://www.inceptionpetfoods.com/try-inception-pet-food/
Hope he gets better, though, nothing worse than when your fur babies are suffering