This is an action that seems to afflict many youngsters. Also, they do like to make a game out of it...see if they can nip you and then jerk their head away before you can correct them.
I would honestly avoid correcting by tapping, slapping, popping, hitting, etc. the mouth, nose, cheek, even neck. Doing so may work for some horses, but any who are persistent at all, will definitely make a bit game out of it..."can I bite and get away before she slaps me?"
When he nips at you at all, I would suggest coming at him like a big scary monster (you don't have to touch him), but back his little butt up hard and fast with your scary person attitude. Even as a young horse he is playing little dominance games...can he bite and get away with it? Can he get you to react? Can he get you to move away from him?
If you watch a dominant horse and less dominant horse, the dominant one often will walk around behind the other pushing them, making them go where ever the dominant horse wants...when the positions are accepted, this is peaceful and no big deal...You are working toward that with your boy, but right now...you are in the establish your pecking order stage, so there will be some scuffles until he accepts his position.
When you make him back up and move his feet AWAY from you...you are asserting your dominance and showing him that as the head mare, you will not accept his little game. If he ever does it when you are loose round pen, pen, arena, your best bet is to drive him away from you, but keep driving him until his heart is really pumping, and he has to think, "what the heck just happened?"
Your level of intensity really depends on your horse, but you really don't want to be too wimpy about it or he will not take you seriously when trying to establish your position in the pecking order. If you do this a few times and your timing and intensity is good...I bet you will see an improvement.
The other thing is that if you are firm about not letting him nip at you (how ever you respond to his games), he might simply grow out of it.
Good luck