I personally think that the whole "open eye" power sounds cliché as hell and completely unattractive for a manga like One Piece.
But it's suspicious that he didn't even get asked about it??? Your friend appears like that and you ask nothing?
I hope we'll get a flashback of how he got it. I fancy seeing Mihawk hurting him during training or something ahahah. But knowing Oda, there just may never be an answer.