If he can swim and fly, let him stay where he is. A foot injury can heal on its own. It will probably be painful for him, which is understandably hard for you to watch. But there are few people sufficiently trained to help wildlife. And the stress of capture and treatment is sometimes more than the bird can handle. If he can swim and fly he's got a strong chance against predators. It's better for the mallard to let him stay with the other ducks.