Well, you just really have to be blunt with your parents, it seems, and let them know that they need to work with him when you aren't there. There are some dogs who are constantly going to challenge their humans, and they are the ones who need constant reminding.

Do they exercise him enough? Letting a dog out to run in the backyard isn't enough. Are they making sure they take him places where he can safely meet others? Many people think you need only to socialize puppies and then they are set. But exercise and socialization are for life.