I am so sorry you lost him. :hugs He must have had something else going on too. It's so hard to know they hide their illnesses from us so well sometimes until it's too late for us to do anything to help. Only way to know for sure would be a necropsy.
Is there swelling in the whole foot or just in that one place like previous picture?
I'd bring him in if possible so you can monitor is eating and drinking and pooping.
Also there is tube feeding which I can give you info on if you want to try. Many of us have had to tube feed and it can...
If you can take your duck to the vet that is one option when I took mine she never even offered to open it but after 7 days on the antibiotic with no change I knew I had no choice but to get it out.
That’s looks very much like the bumble my Runner had like a marble under the skin. I had her on antibiotic for 7 days I finally ended up opening a small slit with a scalpel and popped it right out. How does that bump feel? Hard, soft?