That just doesn't sound like a foxes doing to me. Racoons are notorious for being able to dowhat you describe through such a small opening. Seen it too many times.
Ducks don't "hurt" like we think of "hurt". Most likely he will be fine and live a pretty normal life. You don't have to bring it in the house or spend money on a prosthetic. It is an animal and animals are VERY TOUGH. I have killed wild mallards and wood ducks, even Canada geese with...