This is a tough case, part of me thinks you should euthanize but then again I know that if it were me I would want to try and spare its life.
It sounds like hes a strong one, but ducks arent like people who are able to quickly adapt to situations like this. They're prey animals who cant survive handicapped in the wild.
So my advice to you is to bring him inside (maybe with one of his siblings) and set up a brooder for him. Theres no reason to put him down before hes given a chance. Ive seen many ducks and chickens live without working legs, one example would be Tiddles the Pteroducktyl on instagram. Hes an adorable duck who lives such a wonderful life indoors even without the ability to walk or move his legs.
Give this little guy a chance, but if he doesnt prove to you that he will be able to live a good quality life, then you can take euthanasia into consideration again.