Help with dog training?

Huge problem! We have a 3yo Bichon Frise, sweetest dog in the world to us and our kids. But very territorial. He is fine out in public with other people and dogs, but whenever anyone comes to our door he goes ballistic. This is OK with me (he is a perfect watchdog) except that he got out today when we opened the door just a crack and nipped at my daughter's friend on the porch. I need to IMMEDIATELY train him with some sort of "stand down - it's OK" command, and have no idea at his age where or how to start. Please help, someone who knows dog behavior. We must do this on our own, since I would like for us in the family to be the only ones who can issue the stand down command.