My first lovely big boy Muffin, who sadly died 18 yrs, when he was going down hill, stopped eating, the vet said to get some fish (from the fish shop) feed it without the batter, we all knew he was passing over, but none of us wanted to see him thin, as he was a hunky chunky lad. He ate it with gusto, wouldn't eat anything else,sort of gave me hope but vet's said, he still didn't have long.
What I'm trying to say is try & feed anything (try the fish) don't let him forget why he need's food.