What sort of situation is he in? Stall? Pasture? Is he close to other horses?
Ideally he should be close to other horses (or even another companion animal such as goats, donkeys, etc..) and he doesn't necessarily have to be in a pasture with them, but they should be clear in his sight so that he doesn't feel like he's alone in the world. Horses that are kept secluded have a higher chance of adopting aggressive behavior.
But whatever the situation..I really like Chris Cox's advice for this. This is sort of what he says (for the example I will refer to a stall but it's the same either way):
A horse will start to see his stall as his little domain, where he is the ruler and he decides what goes. What we need to do is show him that even when he is in his stall, he still needs to be respectful and behave himself. You can do this by moving his feet, getting him working. Groundwork, basically. Try sending him around you in a circle on a leadrope (being sure to stay out of kicking range of course!) and having a sort of really mini lunging session or something, being sure to change his direction and have him stop with his attention focused on you, preferably facing up to you. But that's only an example of course, if you have other methods of getting his respect follow whatever you would follow elsewhere, just in his stall. You may even try riding him there!
If that doesn't work, you could work on teaching him to just stay out of your space, but you may have to pop him pretty firmly with a leadrope or stick to get that point across. By no means should you abuse your horse, but if he is doing something as dangerous as what you're describing, you have every right to give him a bite back to tell him that you aren't going to have it.
It's some things to try, but of course different horses have different minds and learn from different things. Just thought I'd put my two cents in.
Good luck =)