have you told your nephew's parents about this? because I think they need to know also that they're son is capable of doing such act. we can't always control what our kids see in media regarding sex. but we can always start the prevention at home. for me, I always tell my son no one can touch your "not-not" only me and daddy when we're giving you a bath. so, everytime his Lola ask to kiss his pototoy he will answer right away "No, Oma!" start talking to your son and explain things to him.its never too late to teach him how to say no when he thinks things are not right.
Magbasa pa