My female rottweiler twisted my male rottweilers leg. . . HELP!?!?

He is limping and it looks kind of swallon, what can I put on it or do? They are 4 months old by the way. My dog is actually walking in three legs. The female rottie is two times his size and weight. He is like the runt born last, so I need help what can I do to help it heal?