By seeing the reactions of others. Treat people how you want to be treated. And it isn't as if he has never been shown kindness in his life. One can choose to repeat the cycle or stop it.
Not looking for an argument but it's kinda common sense??? It's not like he was stuck at home all his life, he must have had interactions with people other than his father and coach which should've told him how to act
let me add: Jaek IS a pos! i just want to see what others have to say!! his behavior is explainable NOT excusable.
let's blame this part as a poor representation cause there's not enough evidence to show his interactions with other people to prove that he learned some proper manners at all. We just kinda assume he grew up with his dad as a role model, but ofc, like everyone has pointed out, he gotta had some other interactions with teachers, friends, at work,... during his childhood.
if no one ever showed you how to communicate, how not be a dick, and the only father figure you had literally yelled at you, you were beat, abandoned, degraded everyday, grew up around thugs and gangs, etc. how could you end up being kind when no one has taught you how to be good?