Spoiler alert:
The parents are clearly lying, and the abuse gets catalyzed by the mother, because of clear principles about the mother-son dynamic. It's very close to the male-female social bonding dynamic, where conflicting biological needs cause each member to constantly insult the other, most often without conscious awareness.
However, let's look at the son, too: If he's a self-unaware juice-squirter who kept the same head from elementary school, then obviously when he goes through puberty, he's going to go spasmodic and reactive. So why should anyone pity THAT person?
From this short clip it's difficult to get a real sense of the clear picture, but my first inclination is to believe the son. He was probably not a perfectly behaved person, but that would never mean he should be getting choked or having a fist raised at him.