We had a cat that came in like that to our shelter. Maggots, gaping holes, and he was in a foster home after being at the vet for a long time. I never thought he was going to pull through, let alone get better.

He actually did pull through, and according to the foster mom who has him, is now fat and happy. This is at least 1 1/2 years later though.

Every case is different, and if there is no way he'll recover, and is just suffering, then I'd try to do something to help him go to the bridge.