Have you gotten at least an A rank for all his orders?
I have S Ranked every customer on every day (Including Captain). Captain maxes out at Rank B for me.
I also have this bug! S Ranked each and every day! I'll try and redo it tomorrow to see maybe it's just me but yeah Captain maxes out at B for me too.
Aside from that I've been keeping tabs on this game for a whole year now! And it was sooo worth the wait I am SO happy to see it finally updated!