{{{{{Beth&Andy&Matt}}}}
If you want suggestions, here are some (spoiler-fonted in case you have already tried fifteen things, and just need me to shush and leave it alone): 1) smelly is good - try tuna or other canned fish, or drizzle tuna juice over his food. Or maybe it is time to go with Fancy Feast, although it is not the highest quality nutrition, it is best of the cheap stuff, and it is very smelly. Baby foods (without onion or garlic powder added) is smelly and is sometimes tempting when proper cat food is not. 2) Nutri-cal [link] works on the same principle as nutritional shakes like Boost for humans - lots of nutrients, lots of flavor, easy to get down. You might have to put some in his mouth at first, but he may perk up and take some independently after the first taste. 3) canned pumpkin will sometimes tempt a cat when other foods are no longer appealing.
Also, sending lots of purr-vibes.