Responses
I remember one of the chapters where Fuuta tells akiyoshi he had it for a little but faked the rest.
Maybe a different translation or an extra chapter I didn't read because I went through the whole series again and it wasn't mentioned anywhere. Akiyoshi just asked (kind of) and Fuuta says "yeah I remembered" which still means he wasn't faking the whole time.
So I was skimming through the comments (because I wanna read this series again but I'm fangirling a bit first.) I don't think Naruse was faking amnesia the whole time, there were some thought processes when he was Fuu-tan for example where he learned pretending to be scared would make Akiyoshi care more about him. Normal Naruse is already pretty manipulative so he would have known immediately.
Then I went back to CH 4 pg53. After the others take Fuu-tan home and they walk in on Akiyoshi... I think that's when he remembers because on this page, at the end, Naruse says "As expected of Akiyoshi" after admitting he remembered, I think he remembered because of "that." Because he saw Akiyoshi getting it on by himself ╮(￣▽￣)╭