surprises me that he went diabetic! What is his story?
I recommend you stick with raw. Much better quality and lot more protein options. The reason people choose FF is because it's cheap and low carb. It's too high in phosphorus should your cat ever have kidney issues. If you want a low phosphorus raw, stick with ones that use egg shell calcium instead of raw bones. Not sure if Small Batch is available near you. Or you can make your own with a premix. When Neko got CKD I switched to making my own for the types of proteins that were higher phosphorus that I wanted to feed. FoodFurLife's EZComplete is one option, easy peasy to make your own.So upon further research, I decided to go (mostly) raw. I do still feed about half canned for the protein variety. But even now I'm thinking of changing to FF pates as so many people on here seem to have great luck with that, instead of the Merrick. Not sure.
I'm near Vancouver, Canada. I got meats at a local exotic, organic meats butcher (5 minutes awayam curious which types of proteins you're feeding and where you are getting them?