Firstly I would have his teeth checked. Have his wolf teeth been removed?
Also I see he is a yearling. Sometimes yearlings go through strange defiant stages, it could be as well that he is just being a bratty little boy, and tossing his head as he is feeling good, but doesn't want to rear or kick up when leading...(my stallion actually does that.)
He could just be going through a phase.....my yearling Tux (now Two) went through a phase similar to this.
I Did the same thing, checked for pain, no teeth problems, so just dealt with it as a training issue, and he stopped doing it after about few weeks
Horses are like chips...you can never just have one