It sounds like the MinPin is asserting placing his stamp on your house and sending out a message to the other dogs. It is essential to deodorize where he has already marked in order to not draw him back to the that spot. In addition try using DAP in the house to help all three dogs feel more chilled. Also, keep the MinPin on a leash attached to your son's wrist. As soon as the dog starts showing signs of sniffing with intent to mark, whisk the dog outside. Treat him in the same way you would a dog that isn't house trained until he realizes he's not allowed to mark in your home.