If it were me (and it has been) I would start by building trust and respect in other areas of handling this horse. The question you're addressing is not just "will the horse do what he's told" but "does the horse see me as a leader" and "does he trust what I am doing?"
Take some time to desensitize him to things, just like Cherie said. If you tiptoe around him now you will have to do it forever; you have not solved the problem you're just avoiding it. I would start with his whole body though, take focus off his head. Throw ropes, tarps, plastic baggies, whatever around his belly and legs using the methods Cherie described. Once he is good with those parts focus back on his head (might as well take care of the whole horse while we're desensitizing).
HOWEVER: Intersperse desensitization with sensitization. It is surprisingly easy to teach a horse the difference between "I am touching you, please stand still" and "I am touching you differently, please move" so start doing this after a few positive desensitizing sessions.
Pet his face passively and, I know it sounds a little hokey, but pay attention to where your energy is. If you're focusing intently on his face that alone is enough pressure for some horses to want to move/otherwise react. (The desensitization business will solve this). Start out just by standing calmly near him until he relaxes (sighs, lowers his head, licks his lips, cocks a leg) watch his ears - one should be focused on you. Pet him on the neck, face, etc. while he relaxes. Then ask him to move - change your posture, wave your hand, put pressure on his jaw with your fingers (common sense please - start microscopic then work up in intensity, especially if you're standing right next to him.) Best to do this in an arena, pasture, or round pen too - he will feel safer (and you will be safer) if there is somewhere to go when things get creepy. The *moment* he moves even an inch, back off and pet passively again. He may try to leave but just ask him to woah (gently) and follow him, petting passively, until he stops moving.
I am normally not an advocate of bribery during training. But a hand-fed treat reward might do the trick. Do your horse the favor of not spoiling him though by requiring an action before giving a treat. They won't beg if they know they have to work for it
Finally, make sure you reinforce touching over the rest of his body (legs, belly, tail, udder/sheath, neck, back, etc.) every day (it's fantastically easy to do while grooming) to make sure he is simply used to you. You might find he has issues being touched in other areas too that are good to find on purpose in a controlled setting than accidentally at a show or on a trail.
Keep is posted!