why did he change his phone number tho?
He felt he was a burden on both of them, and probably also assumed that he was part of the reason why their family is like that. It's a common thing for children in abusive households, sadly.
So I'm guessing that he wanted her to be "free" from both his abusive father and him. He doesn't want to hold her back from being happy (in his mind).
Also like he said, he's afraid of ending up like his father and likely thinks that it won't happen if his mother isn't around, so there's a selfish angle to it as well. Koreans believe pretty heavily in nature over nurture, after all, just generally.
It's probably also why he keeps romance at a distance the way that he does