OK, time to play the horse's game only you get to change the rules without telling him. When he suddenly dives for the center of the ring, don't fight him but encourage him to keep going and if you have to really encourage him, even threaten with a crop then work him in small tight circles, until you feel the resistance going out of him. Keep him at the trot as you move toward the wall again. He'll do it again, so just ride the tight circles again. About the 3rd time he should be figuring it out that instead of getting out of work you make it harder for him. When he passes that point and offers no resistance, do allow him to walk. That is his reward. If you can ride him the next day and see how he is. He may test you. If he doesn't then make this schooling session a short one as a reward, Somewhere along the way he may test you as horses do that but one reminder is often enough to deter this unwanted behaviour.