Spoiler (maybe?)
As per the novel, he hasn't awakened at this point in the story so Eu jae or anyone else obviously has no idea about his rank. He awakens at a later time during one of the experiments that were done him and was found by Jung min afterwards. For Eun Jae who has only seen him at his weakest and then at the highest rank in the whole country, it makes sense that he's so oblivious to his identity. Alsooo no hate on our beloved mc but he's not exactly the smartest cookie in the jar.
HAHAHAHA he indeed isn't the smartest huh. But what I meant with the S-rank comment is Euijae, not Sayoung, I know he's not S-rank yet at that point. I thought he'd be way more perceptive idk if it's canon in this story but otherhunter manhwa have you believing S-rank has superior senses esp sight and smell, surely if that also applies in this world he'd notice how the boy's eye, face shape and even smell is similar
This is slow burn at its finest. Also kinda funny how Eui-jae not realizing who the kid actually was with him being an S rank and alll, he should be way more perceptive than the average person, with his senses heightened and all..... Also just look at that fucking long ass eyelashes bruh we all know who that is