Sometimes you have to teach them to obey you before you hit the source of the problem. Let me explain.
You just get finished with his round-pen work, give him a pat, and then proceed to calmly walk out of the round-pen, and then immediately the battle begins. Or something to that effect.
Try this instead, after you're finished, give him a pat and a small break. Walk him forward a couple steps like you're about to leave, then stop, and quietly back him a couple. Give him a pat, then walk on again. Repeat this until he fully understands that he is to just calmly walk forward, and calmly back away from you. Do that all the way through the gate and past the grass, until you slowly work your way to your destination. It's a positive reinforcement and it keeps him thinking about you, and not the spot where he gets away with naughtiness. :)
That is a very non-aggressive way to approach that problem.
Sometimes abused horses will react aggressively due to their histories, their fear has made them react in a state of defensiveness. Instead of getting aggressive back when he reacts aggressively towards you, you can instead just quietly show him to way to peace. If you have a flag, or even a whip, or anything that is an extension of your arm, you can use it to calmly put a barrier between you and your horse. The moment he acts aggressive, quietly put the flag (or whatever you choose, something that won't traumatize him) to his eye, shake it if necessary. Remove it when his behavior dies down.
Horses like that need a stable, calm and clear leader in order for them to feel at ease and at peace. If you were to observe horses in the wild, the alpha horse doesn't need aggression to gain respect. Horses will naturally follow the one that seems to most level and confident.
You're right in wanting to work with him, instead of against him.
I hope this helps.:)