Seeing the raws, I re-read the manga assuming that he remembered all along and some of the things he says in the present make a lot more sense. When he muses to himself, and when he talks about Jiyoung infront of Hakyung to get a ruse out of him and to watch his reaction. Why would he lie about forgetting that summer, I don't particularly know, but from the raws, he definitely doesn't trust his family and likely pretended to forget so they don't worry about him while he continues to look for Hakyung.
Seeing the raws, I re-read the manga assuming that he remembered all along and some of the things he says in the present make a lot more sense. When he muses to himself, and when he talks about Jiyoung infront of Hakyung to get a ruse out of him and to watch his reaction. Why would he lie about forgetting that summer, I don't particularly know, but from the raws, he definitely doesn't trust his family and likely pretended to forget so they don't worry about him while he continues to look for Hakyung.