"You're the man of the house..."
So when Max shows up at the house, he tells Michael he won't come in unless Michael invites him. This is an old thing in vampire lore - that a vampire either can't enter a home without being invited by the man of the house, or as in this movie, the invitation renders the target powerless.
However - how is Michael the "man of the house"? Wouldn't that be Grandpa? It's his house. What, once you hit 70 you're not a man anymore? That part always bugs me, cause Grandpa was a badass.
The further a society drifts from the truth, the more it will hate those that speak it.