I know how you feel, before I got my first bengal I had a tiger cat, he was 18, he still ate but just continued to loose weight. Finally after putting it off as long as I could I had to let him go. Its so hard to loose them but there comes a time when we have to put our feelings aside and do whats best for them. He's an old man and if he won't eat and nothing tempts him you can't let him starve. Hard as the decision is we have to do what needs to be done for their sake.
I hope he starts eating and gets back to himself but at his age you must prepare for in case he doesn't. You know you are not alone here. Any help we can give you just ask
he is your friend and protector, he will love you unconditionally, you owe it to him to be worthy of such devotion