He could provide his friends statistics on male nurses, it's hard to argue with facts. He could talk about the prestige he has for it because of how important and valuable it is to him. He could also talk about the self-fulfillment he gets from fulfilling his ambitions and desires by his own efforts of becoming a nurse. He could also explain to them about stereotypes and how they are an oversimplified and distorted belief about a person or group. But as long as he has self motivation and personal drive then he doesn't need to rely on group motivation.
