How does he move overall? Is his body stiff? Is he moving with his butt underneath him or all strung out and tense? Reining is about a lot more than just a headset and breaking at the poll, though that is part of it. Once you get his body working properly, his head should fall mostly in place.
That's as far as I'll go, as I'm not a reiner or a professional trainer by any means, and I'm venturing into territory I know little about. Hopefully someone will have more information for you. Best wishes :)