In his first administration he engaged directly with North Korea which has been widely regarded as a Chinese puppet state. The last thing China wants, in my opinion, is a united and free Korea.
He engaged with North Korea almost as an admirer and their leadership is close to China / Russia.
I'm not sure that means much as far as China goes....
Although China's been taking over the SCS, I haven't seen many open hostilities between South Korea and China covered in western news media, almost like SK ignores China's activities for the most part. I don't think there's any chance of a reunified Korea under the Kim dynasty or within 10 years.
edit: forgot to shoutout above's username