Looks like he may have bumblefoot, and his behavior indicates that he has an infection. A trip to the vet, if you can do it, would be good (if the vet is willing to work with ducks).
Aside from that and in the meantime anyway, here is what I would do.
Separate the duck from the others - give him a nice safe place with fresh food and water, a quiet place.
Apply Epsom salt compresses three times a day - let me know if you need more detail.
I would also follow the first compress with a few drops of clear iodine on the swollen parts of the foot. Here is what Haunted55 suggested on another thread:
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Anyways, with the pain, I would say the Duramycin-10 is a good idea and for the bumbles...I'm seeing 5 of them, possibly 6, right near your thumb in the 3rd picture. I use Decolorized Iodine. You can pick it up at WalMart for cheap and that bottle is going to last a long, long time. Let the duck swim in a tub or whatever of warm water, just deep enough for the feet to be up off the bottom. Let them play for 15 mins to a half hour, whatever you can do. You want the skin to soften and the pores to open with the heat. When done, dry them and the foot up and then apply the iodine to the swollen area, not just the top of the lump. You need to paint the whole lump, let it dry and then let the bird go. It's that simple. the iodine is going to go into the skin and start killing the infection immediatly. In a few days, 3,4, or even 5, when you go back to look, you should see brown or black scabs. Soak again, same way as above, peel the scab, see if there is any of the hard cheesy looking pus behind the scab that you can easily get out with tweezers. Once done, dry the foot and paint again with the iodine, dry it, I blow on their feet, they seem to like it and they just relax. Make sure you not only get the iodine inside the hole, but again all of the lump. You may have to do this 4-5 times with bad ones, less with the smaller ones. The whole trick to this is not try to rush the process. Soak, paint, dry and then wait. You do not have to do this everyday and really, could do harm that way. Let the iodine and the body work. At first you will see thick scabs coming off, with all kinds of stuff stuck to the back. This is good as the body is pushing the poisons out. At the end of treatment, the swelling is down and when the last scab comes off, there will be nothing but healed skin behind it.
Last thing...even though we want this to be over with, do not rush it. Using the iodine everyday can cause problems, it can build to toxic levels if used too often. Believe me, you will see better results waiting in between applications and not be risking using too much. Good luck!
. Or, I would follow each compress with a coating of triple antibiotic ointment without painkiller." end of quote
Amiga here:
I am not an antibiotic expert, but if I could not get a vet's help, I would get Duramycin 10 and mix it up and give it orally. One BYC reference I am aware of suggests "Duramycin-10 at 1/2 tbls. per gallon for 7-10 days."
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Also, poultry vitamins especially the kind with probiotics - I would put that in his water once a week for a boost.
And watch him, and give him some words of encouragement.