How do you stop your horse from throwing his head around?

Make the fence taller so he can't. If on a budget a few strands of barb wire will get the job done. Better yet is hot wire, he'll learn after about 2 trys. And if your horse is a cribber, or even if he's not and he's just obsessively resting his throat on a fence, try a cribbing collar. And you may want to observe what;s so interesting on the other side of the fence that has his attention and relocate it. :)