I think his mom (technically aunt) blackmailed him. In chapter 67, when they fought outside the blonde hair dude, Seo Juheon, grandpa's house, the aunt has a picture of Baek Dohu. So I think Seo Juheon left to protect Baek Dohu from his family.
Correction it was chapter 66.
didn't dohu and juheon also have a massive fight or am i remembering things wrong
Yes they did also have a fight. But I believe it was more of the blackmail
Why did the blonde hair dude leave him