I'm not sure what would be best for Scooter, but I recommend you feed him nutrical in a syringe while he isn't eating.. just to keep him healthy. Have you tried straight tuna? The low sodium kind may be okay. These aren't good long term suggestions, but in the meantime, you know.
There are plenty of cookbooks out there with homemade pet food recipes. You may need to whip up something special for the finicky little guy. Also, if he likes the canned people tuna, try pouring some of the juice onto his food. Maybe it'll flavor it.

Oh, and one more thing. I don't know if I'd go rushing off to the vet about it, but its worth asking next time you're in. I'd be interested to know if maybe he has a bad taste- bad teeth, or if there is something funny with his tastebuds or something affecting his ability to taste food.