Anonymous wrote:My dog would simply run away in this scenario and I'm surprised at the number of people who think a dog would become vicious through no fault of the dog. This is not outrageously provocative behavior. Dogs should be submissive.
You clearly have little knowledge of animals (not just dogs). The behavior OP described is aggressive behavior ("he consistently will stick his face in the dog's over and over again, cornering the dog, often out of eyeshot of the parents"). Even the most mild-mannered animal will lash out when repeatedly threatened and when cornered. That's not being vicious, it's being defensive.
OP, I agree with the PP that you must be explicit in regards to how your nephew should interact with the dog. The boy also needs to know if he does not comply, he's likely to be bitten. You should also try and keep him away from the dogs as much as possible.