Cheol is traumatized. His physique doesn't matter if his mental state is a mess when it comes to his family. He is simply scared.
He's been abused ever since he was kid. He's been unacknowledged and compared constantly. He did try to defend himself but where did that get him? More abuse.
If defending yourself gets you hit more then it'd be better to listen to what they say and comply if it means that they'll stop the abuse. That's a trauma response because of the abuse.
Cheol was hoping that if he does every single thing his dad says, then eventually the dad will be proud or even acknowledge him properly.
His survival instinct is basically following whatever his dad says. It's been shown that if he didn't do what the dad wants, he'll get hit and verbally abused.

Man I love the story and everything but y is the ml such a pussy
I mean cmon he can’t even fight his younger brother also? Tsp so much
Still I love the mc