Swollen rooster thigh

by Rolly
(Penn Valley california)

What would cause a rooster's leg to swell to the point that he would stop using it? His injured thigh is about four times the size of the Nola one. It seems like some sort of injury, but there is no obvious signs of trauma. Could it be some sort of internal infection that is only affecting one leg? Should we get some anti-biotics? Please help. I am afraid that he might not last much longer.