Has the foot been worked on previously? Looks like there's a cut in the center of the foot.
I'd start by soaking the foot daily in epsom salts to see if that helps reduce inflammation. Dry the foot, then apply triple antibiotic ointment to the center cut. Wrap the foot and keep him on clean...
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Can you give his foot a really good cleanup, then post more photos?
Often you may find a small cut or poke to the skin once the foot is clean.