I agree with the above posters and have a suggestion about cantering/galloping. First off, I wouldn't canter or gallop him until you have a consistent stop at the other gaits, just for your own safety and so that he doesn't basically become trained to not stop at those gaits, especially the gallop.
With my mare, she gets excited (almost uncontrollably excited) about running, especially galloping, and throws her head up in the air and just has a grand time. However, when she does that, I have zero control. So, when she does that, I use the things I know about her from watching her just be a horse. I know that she prefers trotting to any other gait, I know that she responds to and picks up voice commands better than anything else, and I know she hates being a "bad girl," so she's gonna do whatever it takes to be "good." So, when she decides to take off, joyfully of course, I let her run a little bit cuz I know she's not being "bad," she's just happy, then I try to stop her by sitting down and checking and releasing on the reins (pull back-release-pull back-release, really quickly), and when that doesn't work (which sometimes it does) I growl at her and say "LACEY!!" really loudly (I just do that to get her attention back on me, growling to her means "Bad!" but her name just means her). Usually that does the trick and she comes right back to me from "running land" and then responds to all my requests for slowing. I even did that in a rope halter and it worked like a dream.
Basically, I guess the moral of this story is: figure out what motivates your horse the best and use it. Also, perhaps train him to respond to a word for stopping and going down to the next lower gait (you can do that on the ground while leading him and lunging him). Then, you can use it in conjunction with physical cues when you're in the saddle and have multi-faceted control, instead of just relying on your own power when your horse doesn't want to stop. With Lacey at least, word cues have come in SO handy. It's great when I'm riding bareback (just one more example, lol!) and I'm losing my balance so then I get scared and freeze up, making any sort of physical cues I can give non-existent, but I can tell her to slow down and stop with my mouth and then I don't fall off!
Good luck! You'll get there if you work hard at it! :)
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.
Last edited by Wallaby; 09-12-2010 at 12:59 PM.