Hmmm..... Maybe it was how he was hit on the muzzle. Some horses are ok with it, some arent. If there was a lot of swing and if he was punched instead of thwarted or small smacked, then that could have done it. I had to smach my guy as well to get the biting to stop it was bad. But I didn't do a big swinging smack, just a "hey don't do that" kind of thing. I would also smack him in the body if convenient to do so.
As far as fixing the headshy thing, I would start by slowly rubbing your hands all over his "safe zone" on his body. Sloooooowwly sloowwwly slightly bring your hands up his neck, as soon as you notice the SLIGHTEST discomfort, return your hands to the safe spot, and repeat. You will have to do this on both sides. When you are able to get a little further without him flinching or what not, try keeping your hand there for a few seconds, then slowly go back to his good spot. And repeat...that's what I would do. Patience patience patience. He has to trust you again. You can also do the above excercise with one hand and keep another hand on the safe spot so he feels grounded and comfort. Also make sure you are in an open area and he isn't tied too tightly if at all.
Sis was incredibly headshy when I got her she would rear up, jerk back anything to keep you away from her head. I started by working backwards with her....she was ok to pet her butt, back, shoulders and neck so I basically started at the backside and slowly worked forward till she started getting nervous then backed off. It took a lot of doing but now I can pet her on the muzzle, rub between her ears which was a Major issue now with no problems.
I strongly discourage giving a horse who had previously been a really bad biter, treats...atleast not out of your hand...you could wind up back where you started...with a bad biting horse.
You're just going to have to slowly work on getting him used to you touching him all over again...as others have mentioned, work from 'back to front'...rub him in areas he doesn't mind, and work your way foward, till you are at his head; always back off to a 'safe zone' (a place he doesn't mind being rubbed), before he acts up, or pulls away from your touch. It will be sort of an approach and retreat tactic...you'll touch for a few seconds, while he tolerates, and back off before he can react.
I also advise against feeding treats- that might put you back where you started. In addition to the advice already given, don't force your touch on him. Give him room to move away as you work towards his head.