Ducks can recover from awful injuries.
I would give him time in a lukewarm tub of water, time to just float and relax. Watch closely to make sure he can manage without trouble. You will also be able to see how he can move his legs without weight on them.
The water cheers ducks up, takes weight off the legs, improves circulation and gives some light exercise.
I would give B vitamin supplements - B vitamins, I am told, help the nervous system. So hydrotherapy (bath time) and the best nutrition you can manage, including the B vitamins, TLC and time may do more than you can imagine. I would give the guy every chance.