I can't see YT from work so will just offer up this until I can see it later tonight:
Has she tried riding him in a fly mask? I ask because it is possible you're dealing with a photosensitivity issue. If the head tossing is reduced or eliminated with the mask, this is most likely the issue.
Alterantively, it would be worth riding him without a noseband on the bridle to see if that has been pressing on a nerve.