When I was into Jujutsu way back there was a response against headbut that I have brought with me and aikified
Since the headbut is really quick and close by it basically means two things
1. Yer' to close mate! Maai mamn it!
2. Since you are so close there is little room and time to tenkan,
so... shove your (e.g. right) lower arm between him and you so that the whole lower arm has contact with his torso (you right elbow at his
right nipple and your tegatana at the left(his left) side of his neck. Its diagonally aligned so to speak.
This will break his forward movement and is fast enough so that you have the chance to have time to do it. After this you can do what ever you want but I usually go for a koshi nage. It's nice if you can do it without actually completely stopping his forward momentum. If you can't then you still have stopped the headbutt from reaching it's target.