Something with a really strong smell, like sardines?
I would also definitely find out from the vet if the thyroid medication needs to be changed or adjusted, in case it is responsible for the lack of appetite.
Also ask your Vet about B12 shots, that's helps to stimulate appetite. Buy some baby food, beef or chicken, just make sure they contain no garlic or onions . You can use a bit of the tuna water from the cans to pour over his food or dust top with some crushed up cat treats...none if these are recommended for long term...only meant to help temporarily stimulate.
Poor kitty just maybe telling you he feels it's his time too, a good discussion with your Vet can help with this very hard decision...
UPDATE - We had to put him to sleep. I guess I knew it was his time. I didn't want him to suffer any longer - he could barely walk. He lived a long life 18 years - had him since he was nursing. The shelter was going to euthanize the mother and her newborn kittens and I convinced them to let me take all of them against their policy. I kept two and found homes for the mom and other kittens when they were weaned. My boy was the biggest, he was huge next to the other kittens nursing.
My other cats have been looking for him this morning. They loved him, kept him groomed during his final months when he was unable.
thanks for your support.