you said and i quote "the son isn't any better" that literally means he is the same as his father if not worse. and as for not doing anything at all you're either really bad at context clues or just not reading at all. the son is trying to take over the mansion and the staff by marrying that woman. his goal is to make yeonho an employee of his rather than his father's. therefore saving him as he will have more power to keep the grooming, rapist pedophile away from yeonho. there isnt really anything else he can do, not at his current level. pay attention to the story it's so obvious.
It's a relationship and it is also about him and affectim him He felt betrayed, because a) the person he was close with and crushing on was having an affeir with his douchebag dad b) he kept it secret. You also have to consider that he didn't know that these things were happening without consent, so he felt cheated on.
Now, he's trying to save Yeonho by marrying that woman. idk about you, but he's def the better option especially considering that the dad is a pedo rapist
the father is trash but the son isnt any better, he just keeps making everything about himself and NEVER takes into consideration the hardships yeonho has been going through since he arrived at that house, he just complains and acts as if everything was yeonho's fault .... sorry but he's just a spoiled rich brat who can't get himself in other's shoes, not even for the person he apparently "loves"