Great advice!You may want to post on the' duck threads' and get their opinions. Maybe the others bully him when you aren't around. That could be why he reacts when they bump him or come near him. Not pain necessarily but fear of them.
Has he had a chance to swim in bathtub or other? Generally they do better on water because their weight is supported, and any swimming motions may soothe his leg. I'm guessing here, have never been around ducklings.
I have read about ducks that lost a leg and they do quite well on water. Wondering what a experienced avian vet would say. Maybe an amputation would be easier to adjust to and hopefully relieve pain, after recuperation.
Start with a kiddie pool