Well, leg mites are easy to deal with. Coat his legs with oil, vaseline or spray with Pam cooking oil to smother the mites. Usually, those result in raised up scales. Red, inflamed legs can be fungal, too, but that might have nothing to do with this. He could be reaching sexual maturity. Treat...
Are you positive it wasn't like that the day before? Or maybe you just didn't notice? What breed and how old? Mature roosters often get a reddish wash down the sides of their legs. I see it in the BR, Buff Orp and the RIR roosters all the time when they reach maturity.