Cause is a loser like him and he knows that
I think it was a reminder of his insecurity. Since the father is the youngest son aswell. So Cheol being the oldest was a trigger for his father to take out all the resentment he had on his older brother. Everything he couldnt do to his brother he took it out on his son in a way. So by saying he's "just like jungwoo" the youngest, it was like reminding him that he always had this competition with his own brother where he felt the favoritism went to him. If anything the father just pitys himself and has so much built insecurity of being in the shadow of his older brother that he won't allow that to happen to his younger son.
But that's just my thoughts loll
why was the father mad at the fact that he was like jungwoo?? isn’t jungwoo his favourite