@Mangareader Pretty sure he hit him in the stomach, right below the ribcage.
@Dreadkiwi He's all bones and zero nerve endings. He should be able to headbutt someone anywhere and it wouldn't hurt a bit. Also, when headbutting someone else's head, you need to use your forehead or something to target a vulnerable spot, like the nose, and not just any part of your opponent's head. That's just asking for you to get hurt as much as the other guy.