Spoilers.......
No. He's the younger son. the other guy is the older one, he was sick so the family thought he would be useless so they just erased him and said they only have one kid. Hajin replaced the brother's identity and is actually younger than everyone. The family switched places with the brother, he goes to school with Hajin's date of birth and hajin goes to uni with his date of birth.
BRO WTFFFF
Ohy gosh are you serious. So your telling this kid got forced into becoming an adult with out even knowing. Nahhhh. That's sad.
hajin is adopted ?