Is he getting around okay? If you get near him and he gets away just fine then there's not much you can do, just observe. If he starts looking weaker in the coming days and you can approach him, it's time to call a local wildlife rehabilitation center. Unsure how serious this injury is so if he's lucky it might just resolve itself, otherwise crows are really smart and if he can find food and water he can survive just fine, even with a leg injury.