As Cherie said, this is fast becoming a game to him, and is much like young horses will play, bite bite bite, until one really bites hard or kicks out...the smacking is basically playing a game. PLUS he sees you DOING something.
Hence the head raising going on.
Either do the horseshoe nail thing, where the nail tip only is poking between your knuckles on whatever hand is closest to the head, rest of fingers/thumb are in a fist, in other words, if you are cinching up, your left hand will be closest, if you are grooming his off side, your right hand will be the closest to his head.
Standing at girth area, ignore horse completely as far as he knows, your eyes are on what you are doing and face is not looking at him, when he reaches to bite, casually move your hand so that he runs his mouth into the nail tip.
Do not give any indication that this has happened at all. Not by voice, gesture, sounds, fast movements, nothing. You are merely moving hand as you would to move fly away from your cake type thing. Lazy, slow, no big deal.
If he bites when near his head, either arm yourself with a nail as before, or you can use a stiff brush, bristles out, on flat of your palm and when he moves mouth towards you, let him run into the bristles. You are to exert no force, nor make any sign that you know this has happened. It needs to be as if you have a prickly force field around you, and that you are not doing anything to cause it.
You can also use the point of your elbow, both at girth or head or wherever, merely bend your arm and raise it when he begins to move head, and let him run jawbone into your elbow point. Again, do this slowly, and do not look at him, or make any sign you noticed it. And no laughter or talking either.
That said. If you are prone to be always messing with his face? Quit it. That will lead to biting faster than anything. No stroking of muzzle, pulling at lips or anything of the sort.
But he is showing you dominance at this point, and it will get worse if you don't stop it.
The trick to this, is to not ever let him think you are actually doing it, but rather he is doing it to himself. That is why it is imperative that you do not make any gestures, or say anything. Just a casual movement of hand or elbow.
Horses make me a better person.