Definitely agree, round pen him.
Your horse isn't giving to pressure. First teach him to give to pressure in a rope halter. Stand by the shoulder and take up on the lead just enough to put a tad of pressure on the nose to get him to turn his head an inch. If he doesn't "give" with the light pressure after a few seconds, increase the pressure and hold. Never release the pressure until he gives then release it totally, throw the lead at him and praise. Once he will give his head to you an inch, try for two, then three and on and on until he can touch his shoulder. Do both sides. Now the horse knows how to give to pressure and you can easily teach "back" because he knows that when he feels pressure he is to move AWAY from it. Pressure on both sides of his nose will mean, back up. Do it the same as for each side. Hold the pressure on, lightly at first (because you want him to respond, in the end, to the light pressure) and only increase when he doesn't respond in a few seconds.