Bro, I am not gonna repeat the same shit. I mean no offense but you're either deliberately acting dense or you have no clue what you're talking about. I'll try one more time.
As I've been saying, he knows he's immortal because the Gorosei literally told him. But he was still skeptical about it because he hasn't experienced it and people generally think it's not possible. So when he was finally regenerating after getting stabbed, he got the confirmation that he really is immortal.
Bro, wtf? I literally posted the page in which he says only Loki can kill him which happened after he experienced his immortality directly.