How old is he? Do you know if he was vaccinated for Marek's disease as a chick? What are you feeding him? Do the pads of the feet look swollen/scabby at all (a sign of bumblefoot)?
Three possibilities that come to mind are Marek's Disease, a vitamin deficiency, or injury. Marek's disease is a common, viral disease that affects mainly birds that are unvaccinated and under a year of age. It causes paralysis and tumor growth, among other things. Unfortunately, Marek's is untreatable. All you can do is try to prevent it either through vaccination or breeding for resistance.
A vitamin deficiency is also possible. In case this is the problem, I would put some poultry vitamins in your rooster's drinking water and make sure you're offering him high quality, fresh feed. You could also try giving him a few drops of Polyvisol infant vitamins if you can't find any poultry vitamins.
Injury is another possible cause. Many injuries can be resolved through rest, supportive care, and as little stressing as possible. I would isolate (if you haven't already) your rooster in a clean, well bedded place. Put feed and water within easy reach, and don't give him a roost. If given some time, any leg injury (sprain, etc.) should heal.
Good luck!