How old is the rooster? Does his leg look broken, or is there any swelling or bruising? Had he been picked on or hurt by another rooster? Can you post a picture? Broken legs and sprains are very common, especially in young cockerels who can be flighty. Resting the leg for 2-3 weeks with confinement to a small pen with food and water may help. Splinting can help, but only if the bone is broken, and if the splint is put on correctly. Some leg injuries cannot be fixed, especially if they are fractures up high on the thigh. Here is a good article (look on pages 17-21) about splinting legs in birds that shows the leg anatomy: