No, I don't think that's bumblefoot. The joint appears swollen, so you may be dealing with internal infection.
Carefully feel up the leg and around the torso. Did he get some fishing line wrapped around the leg?
I would bring him in, get him into lukewarm clean water deep enough to float in. Maybe half an hour in the tub.
I would follow that with an Epsom salt compress (2 tablespoons E.s., half a cup of warm water, soak a clean rag in that, wrap the rag around his leg and foot for several minutes. Then dry the foot and either slather it with antibiotic ointment (without painkiller, which is toxic to ducks) or dab a little iodine on the sores and let it dry. Do the iodine once every three or four days, not every day. I would do the ointment after each soak, if that is what you choose to do.
Keep him in a cozy, clean place with fresh water and food at least overnight. Keep him in closer quarters so he can rest.
If he has internal infection, he needs oral antibiotics. It would be best for a vet to see him and guide you and I understand that is not always possible. I am not a vet, I can only share my thoughts with you.
I would give him vitamins with probiotics and electrolytes to boost his immune system. The sores look like they could be abrasions, but it is hard to say. Could he have gotten stuck for a while on something?
In any case, twice-daily baths followed with Epsom salt would most likely help. Seriously think about antibiotics so that if that doesn't make good progress in a couple of days, you know what and how much to give, if you decide to do so.
