I don't know anything about ducks but, I think bumblefoot affect the bottom of the foot. Please go to "Other Backyard Poultry", above. You will find threads for ducks, geese, guineas etc. The duck
people surely know much more than I do.
You can also post at "emergencies, diseases, injuries, cures," thread - show good photos and all details - for instance how long has he had the bump? did it show up just recently, or has it been getting larger over time, etc.
I hope someone will be able to diagnose the problem and provide treatment options.