Can i have some spoiler!!! Did he knew it was miji since the day he met her and he just pretended not to know…
Ho Su is portrayed to be very perceptive from the beginning. Able to tell them apart in highschool etc. Episode 6 confirmed that he knew all along, probably since Episode 2 when he called her out for being Mi ji. He also confirms this when he talks about keeping secrets for people because everyone has their reasons.