have u looked to see that he doesn't have bumble foot?
is he walking properly?

You're going to need to grab him and examine the foot. Try catching him at night, after he's been asleep for an hour or so and can't see in the dark.

Once you have him as auryn says, check the pads on the underside of both his feet for signs of Bumblefoot--it's a swelling of the pad and if it gets bad enough, an open wound forms. Because the wound is open and gets packed with dirt, it tends to be a chronic condition and the bird limps. Read up online for more info about it.

Also, check each of his toes for injury. It could be that this is just a coincidence. If you find an injury or wound, you can clean it out using warm water mixed with peroxide, and place some Neosporin on the wound. You can't really wrap a dressing most of the time, as chickens will pull it off. After that, place him in a wire bottoemed cage until the wound heals, so it doesn't get packed with dirt again. While you have him in your clutches, check him all over for any other health issues--mites, dirty vent, underweight, etc. He may as well get his money's worth. :)

If you find an injury, think about where he may have gotten it and check over your chicken areas for anything that might wound--broken wire, etc.

Good luck!

Velvet ~:>