did he really expect to find true love from a cougar and have a happy family with a woman a few years younger from you
Obviously not. The flashbacks while he was drugged likely point towards him being coerced into this "relationship", since he never even intended to go for round 2 with his clients, got beaten up, was shown begging before Rosa's mother (and previous client) and kept apologising to whom he thought was her for not going 2.
That or he felt like he had no other choice/that was his best choice into fleeing from a worse or poverty driven life. He said what he said to Rosa because he wants to pretend he's living a normal life and to leave the past (including his and Rosa's) behind, but clearly she's having none of that (in the most problematic way).
He's also echoing Rosa's mother's words (in the beginning of the story her mother mentioned her feelings being different for Yeonwoo, like true love, if I'm not mistaken). So it's safe to assume he's going along with it or trying to convince himself of it to secure that wish of his.
He looks like a whole fking wall in that one panel why is he so gigantic… Anyway idk if he grew up dumb or what but those mother and daughter are clearly batshit crazy bro-