Tell him

Honestly, at this point I want him to tell the truth just because I want to know what exactly he is trying to confess. At the beginning of the story he was accused of being a scam artist but scam artists promise you something and then never deliver but still take your money. He did the exact opposite. He did everything he said he would do and didn't take any money from him. If Seunghyun said something like "I scammed you" the only reasonable answer from Hojun would be "no, obviously not".
And confessing to not being an actual sex teacher would be weird as well because sex teacher isn't a real job anyways. He's also never claimed to be an experienced sex teacher or anything like that. He just promised to improve someone's sex skills with a personalized plan, executed through sexual encounters.
Making a confession concerning the motivation could end up making Hojun look bad. Because let's not forget that they were both drunk and not serious about it at the very beginning. But when Seunghyun answered him, still taking him seriously, he decided to give it a shot. Hojun never confessed that either and he doesn't feel guilty about it. Blaming Seunghyun because he did the same thing he did doesn't strike me as something Hojun would do.
The only potential for a little bit of drama would be that Hojun gets mad because Seunghyun wasn't communicating his feelings (feeling guilty) properly or because the ex is manipulating him.
Overall, there really isn't a lot of room for a confession/reveal that causes a big argument. Most scenarios would either make Hojun look bad or stupid or would be unrealistic which is the opposite of what I want from this series.
Fingers crossed that it doesn't suddenly shift the tone just for the sake of drama.