Ml doesnt know that this is a body swapping situation. He thinks the mc is a completely different guy who just happens to look sorta like song iheon and is impersonating him for some dubious reason.
I mean it makes sense that if you KNOW your rich friend had been replaced by a lookalike, you'd assume Evil Twin before Freaky Friday
Looking at this chapter, it doesn't seem like he has much feelings for the old Yi-Heon. The fact that he called the old Yi-Heon "that kid" implies that they are not very close. And he himself said that he likes the current Yi-Heon better. He even tried to forget about the old Yi-Heon and wanted to act like nothing happened. But right before he fell asleep, the image of the old Yi-Heon flashes back into his mind. This tells me that his desire to find old Yi-Heon isn't about how much he cared for old Yi-Heon but has something to do with his innerself. Perhaps he feels like it's his fault that the old Yi-Heon is gone. Or perhaps his felt sorry for old Yi-Heon. Regardless of the reason, it is clear that his conscience won't let him forget about old Yi-Heon even though he himself wants to. That is probbably the reason why he wants to find Y-Heon. To clear his conscience.
Does anyone know why he wants the OG guy back in his body? And not MC?