The stepdad punching Lewis' son

Why when Lewis AND his ex told his son to get his stuff, they ALL let the new husband Todd stop the boy from leaving with his father, Lewis. Since when would a step parent be able to tell two parents what they can and cant do with their own child? I also felt that Todd punching their son was a green light to beat the *beep* out of Todd, but let him know that if he called the cops they would press charges on him for child abuse. But i know the scene was written just to show how Lewis was a *beep* up, i guess. This scene just burned me up.