How do i help someone who is scared about wet dreams?

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I have a friend who is phoning me all the time and talking to me about him being scared of having a wet dream and getting someone at his school pregnant. I don't know how to talk to him about it. I try telling him its not possible, but he is the type of person that like to be really really sure. Well, who wouldn't want to be sure about something like this? Please help me, I don't know what to say to him.