Okay, the only part that I remember is that they were swapped at birth accidentally. They're still young, maybe a teen ager, or 17/18. When they get back to their original family, the fake young master (MC) family is poor. MC feels like an outsider because his bio mother keeps talking about the real young master or comparing them. MC then left his bio family.
Cannon Fodder Fake Master Was Stunned After Being Reborn. MC is reborn in the past after his mediocre life and death. MC was kicked out of his rich "adopted parents" house. They prefer their real son, never liked him anyway. His "bio" parents treated him badly. His poor “biological” parents are not even his "bio" but they are hiding it. (he was kidnapped from a hospital as a baby) I think real son of the rich family also knew about it? I think he tried to prevent him from finding his real family.