So the mc was reincarnated right? But why is he so dumb😭😭

imo, it is due to his illness—and the fact that the recovery process is still ongoing, thats why he's innocent

Is it like he went to childhood?

most likely yes (??) tbh, the OG Nox isn't really explained well... he's just portrayed as an "unstable omega" or a bully who uses his pheromones to torment Yuri— and current Nox too.. we know nothing