Try keeping him busy with little things like side passing, listening to leg yields, doing lots of transitions, and riding along the quarter line, instead of on the track. You can even be changing the pressure on the reins by taking and releasing. Finally, do LOTS of circles. What I find really works is making a big circle, medium, then small circle, then spiralling back out. I also love to not just circle at the end of the arena, but rather do circles from the middle of the long ends. What ever you do, always keep your horse busy and focused on something so that he doesn't have time to be switching up his gaits. Also, if you find that he's spooking in one particular area, walk him by first. Let him get a good long look so he can see there's nothing there. From then on, ride with confidence and a strong leg, seat and hands, and don't let him look at that 'scary' area anymore. If you ignore, he will learn to ignore it as well. Good luck!