Wookie,

It is how you carry and present yourself to them. He is challenging you and dosn't feel that you are the more dominant figure. You have to be on your game so to speak, teach the horse that you have a bubble so to speak around you and he is not welcome in that bubble unless invited. Use your arms like turn signals (on your call) Warning him that he is to close and if he comes into your space you bump him but in a way that he thinks he has ran into you. It is hard to explain, wish you were here could show you. But you need to catch him before he is in the process of nipping. Let's say that you are standing in front of him at his pen, as soon as he just slightly tips his nose at you, you bump him back. He needs to learn respect and working on his ground work (round pen) and manners (keeping space, etc.)will benefit him. It is long hard consistant work.

Good luck