Awww poor sweet kitty! I'm glad he's had some more tuna chicken soup! I'd prioritize keeping him as hydrated as possible, often that can perk them up and increase their appetite so even if it's gotta be tuna juice that helps! There's a bone broth that's made for cats now from Nacho brand though it doesn't come in tuna (chicken, turkey, or beef.) Sometimes when I have a sick kitty ill mix that in with water to encourage more drinking, same concept works with tuna juice really.