Dont worry, i already found out how to do this... You need to finish all girls content lmao.As far as I know, after you meet him in the school you just have to go back home and Karen invites you to go shopping with Karen and Sun.
This game suck, a sandbox but you dont have guide to do this chained relationship simdate. If the devs doesnt wanna give us guide, just dont make it hard for us to continue 1 relationship