My dog attacks only my nephew, and no one else?

it's probably a good idea to consolt a vet, or even a dog specialist that deals with dog's behaviors, has your nephew ever stared your dog right in the eye, that is a way of telling another dog:FIGHT! my best idea for the sake or your nephew and your dog, is to make sure that he is all ok by the vet(shots, and making sure there is no problem with your dog such as maybe a sliver, or even some sort of infection) and also to consolt a behavior specialist about your dogs "relationship" with your nephew. Hope i Helped :)