I had the same problem with my 2 year old. Horse's minds work in weird ways. In a herd, if a horse can get another horse to move his feet, the horse is considered dominant. (Ever seen a horse put his ears back and the other horse scurries away? The horse with the ears back established his dominance previously, and now is able to get the other horse to move his feet easily). So in the horse's head, if YOU make HIM move his legs, he thinks he's supposed to respect you.
So if your horse tries to nip at you, back him up
. Obviously he's not going to all of a sudden bow down to you, so make sure that you STICK WITH IT! Do not get lazy, because you'll have to start all over! You can vary between backing him up and yeilding him to the side for variation (but don't make him walk forward, as they do this anyway so the other two are more effective). Once you establish dominance, the horse will NOT nip at you because you are now his "herd leader"!
I also recommend this for people's horses who DON'T bite too!! It strengthens your relationship!