Thank you so much for sharing this.I am assuming that a vet is not an option at this time either due to finances, availability of a vet that will see ducks, covid-19 lockdowns etc.
So... if he were my guy, I'd soak him in a tub of very warm water (if you putting your elbow into the water feels hot but not too hot, then the temperature is right), if you will be closely supervising this bath, then add Epsom salts to the bath, but if not then don't. Ducks tend to drink where they bathe, and the salts will give him diarrhea. Allow him to soak for 15 to 20 minutes in this hot bath, then with an old terry cloth rag, gently yet firmly rub downward from his legs to his toes and the bottoms of his feet in an effort to pop the scab off of the bumblefoot. If successful in your effort to open the wound in this manner, you would then apply pressure with your thumb(s) and fingers to the opened wound as you would a cyst or blackhead, applying the pressure from underneath the pus pocket and guiding it towards the opening (that you just made). If the wound doesn't open in this manner, try bathing him again the next day or move on to surgical means to open the wound.
After expressing all of the hard, cheese-like pus in the wound, rinse it well, pack the wound with triple-antibiotic ointment without pain relief (neosporin comes to mind), put non-stick gauze over the wound, then wrap that onto his foot with sports/vet tape (sticks to itself great, doesn't stick to his skin.) Athletic stores sell this tape MUCH cheaper than veterinary supply stores, same stuff, different marketing.
Loads of articles on this forum and videos on youtube showing in more detail how to proceed, but thought an outline might help you.