As he was driving near a (you tell the type of the place where you and the person you’re telling the joke to are; school, residential area, restaurant etc.) when he found the third son.
But this time, he decided, just as an extra precaution, to make sure that this boy is the prostitute’s son.
So he approached the boy, took a deep breath and asked him…
Now you start laughing. And then you say, “Never mind. It’s a terrible joke. By the way, have you heard it before?” If the person says no, then you ask, “Really? You haven’t heard it?” If the person says no again, you slap his back and say, “Found you, you son of a whore!”