Why, for the first time, is Paul no longer afraid of Erik and Arthur?

Paul stands up to his father and refuses to name Tino and Victor and the boys who attacked Erik. When Arthur and Erik later approach him by the wall, Paul feels no fear..... the same feeling he had when he stood up to his dad. He tells Arthur and Erik to try and hurt him. But as they face him in the dark with swollen eyes and faces, he realizes that there isn't anything to fear about them at all. They are actually totally weak: their threats are empty. He tells them that he saw them kill Luis. He says there were other witnesses too.