The horse is probably taking it personally because he is the only one in the pen and all the pressure is going directly at him. Try putting another horse in the pen with him, one that already knows how to turn nicely that can act as a good example, then there isn't so much pressure on the one horse. It created a more friendly learning environment (and no it will not make him heard sour but rather better socialized). You will not have to worry about the horses going after each other if they are given something constructive to do, you could even work them different directions in the pen at the same time if you needed to. I used to do this a lot more before I sold my round pens, eventually I'll build another one. It makes riding in a warm up arena or anywhere with multiple horses a whole lot better later on.

Step back and take a look at your contribution to his defensive behavior, are you continuing to push or nag when he is already moving out for you. Are you staring at some part of him and causing a brace in his body? If he is backing off the rail 10 or 20 feet to kick you that is an aggressive behavior and something else altogether. I wouldn't be volunteering unpaid time to work with a horse like that, I have in the past but not without medical insurance. I will say that if a horse is being truly aggressive and you go in there to "wail on his back end" you are knowingly putting yourself in a position to get hurt bad, you are encouraging the horse to fight back if he isn't going to be intimidated by your attack. Be careful with that, I received the same advice from some well meaning peers when I was learning to start horses and when I followed their advice I got the hoof prints to go along with it.

"It's really quite amazing what a horse will do for you, if he only understands what you want. 
And it it's also quite amazing what a horse will do to you if he doesn't." ~ Bill Dorrance