Dude, Cheol is not his uncle’s son. The part where his uncle said “He’s my first child” or something, he just said that because he feels like he’s the one who raised Cheol since he was young. That’s what I have understood, since if he was indeed the true father then he would have the custody or power to protect Cheol.
Aah i get it now.
I believe you did misunderstand the context. Cheol considers his uncle as his Father, the same way as his Uncle sees him as his own. His uncle feels pity for Cheol knowing that he was abused by his own brother, so its like he took it upon himself to be the father figure for Cheol. I don't think his statement supports enough to claim that Cheol was his, when all through out the story it was clearly stated that Cheol's uncle is Cheol's biological Father. They simply have more paternal bond than Cheol is with his own Father.

The cmts talking abt the father abusing their eldest child. Didn’t we know already that Cheol is the uncle’s son !? I think that’s why he hit Cheol.