Did you gently check his foot and leg for any swelling, heat, or visible signs of injury? You need to eliminate the possibility that he has cut himself or has any sort of infection on or problems with his foot. You may be able to see or very gently feel if he has broken something. If they have a broken leg or toe, you will need to splint it. If he has sprained or broken something he will have swelling that is not hot. Heat usually indicates infection and will require antibiotics: You do not want to give antibiotics unless you are fairly sure its infected though.
I've had roosters end up limping quite a few times over the past year with zero signs of what happened. I was able to see what happened in only two cases: One started limping after being chased by a more dominate rooster for an extended period of time and the other was chased over uneven ground...this time by my cow?
If I see no visible signs of injury, the area is not hot or discolored, and the limp is minor I will leave them alone and watch for a few days to make sure it isn't getting worse. Usually, it just goes away on its own.
If the limp is more severe I usually put them in the hospital cage to let them heal for a few days up to several weeks. Try to make the cage a small, but not cramped: The point is to limit their movement, not completely restrict it. Just keep their bedding clean, give them plenty of food and water and check them daily (can just watch...don't necessarily have to pick them up every day....it will stress them out). If they start doing better you can move them to a larger area, but still restrict their movement until they are walking much more normally. This has worked well for me and even my most severely injured bird (attacked by dog...leg bitten) was able to walk fine afterwards; although he had a slight limp the rest of his life.
I have an injured 9 week old guinea right now that suddenly acquired a severe limp and was extending her wings to balance herself as she tried to walk. She is now in a hospital cage to limit her movement and keep her off that leg/foot. Not only will the leg rest, but if they are getting picked on, it will remove stress as well. I made a way for my guineas "flockmates" to access the area she is in during the day and they all stay pretty close by. She is noticeably calmer when she can see her unusual little family (my barnevelder rooster, an easter egger hen, and my other 9 week old guinea). Each day she seems to be trying to put a bit more pressure on her foot and the swelling is beginning to go down.
Hope this helps at least somewhat.
