So are we ever going to get a answer about that the dad called about? This series is to sweet but kinda has me worried it could take a turn.
It won't come up until the final arc of the series, because the important part for now is reestablishing Yuichi's bond with Sumi and the shifts in themselves and their relationship from where it was before.
Only when Yuichi begins to accept that they are important to one another, that he
likes Sumi romantically,
and that she likes him as well, will we get that reveal.
Because he has to be invested in Sumi before it can be shown that she might have to leave him; otherwise there's less emotional impact to his character, and less onus for him to try and keep her around.