This is going to sound daft, but my oldie (Pursley, 15) has "down " phases where she can barely move off the bed and eats next to nothing for days. The latest one followed her annual booster, which I expected, but the vet checked her thoroughly then and she was fine if a little arthritic in the back end, but the episode prior to that was unexplained - similar to Ping.
I was heating through chicken soup - the Covent Garden stuff, and thought she might like a drop. She did. She had a good few spoons full before starting a face wash, which was something else she had been neglecting.
Now I know that onion powder and garlic is bad for cats, and of course there will be a trace of these in the soup, but she was eating so little of her own food it was becoming a case of getting anything inside her before she made herself ill, and she immediately seemed happier.
The power of cooked chicken is well documented in this household and now chicken soup has been added to the list of miracle cure-alls because this seemed to bring her round like nothing else could. May be worth a try if you have a can of soup in the cupboard, or whizz a bit of cooked chicken into a puree for him?