He obviously doesnt like having his head touched... have you tried stroking or scratching his shoulder or the top of his withers? Most horses like that. Once he learns to trust you touching his shoulder you can move up to his head. if he bites you when you are stroking him flick him on the nose as soon as he does it and saying loudly NO. I did this to my filly when she went through a nipping stage and it quickly broke her out of the habit. Obviously its a bit different for me because my filly was very used to me and obviously this horse doesnt know you. you can but try!

good luck!