The look on his face when he starts calling him out shows how in the dark he’s been about his own son. He didn’t even know how little he knew.
Side note, he calls his enemies techniques cowardly, as if they never trained to beat him or get stronger? What a self centred and unobservant man. He tries to fit the role of a hero to the point that he’s not a father and won’t even recognise the effort of his enemies. His view is that anything you do is illegitimate or ill gotten if you’re against him.