This may not apply at all (and the first thing I would do is to really work with him in a contained environment, getting him truly listening to you, etc) but when my mare started the "LOLz, I'm going to bolt unexpectedly with you [or refuse to stop, or dance around, you know the drill] and since you have a gentle bit in my mouth=HAH."-thing, after I worked with her a bunch in an arena and saw improvement - until the next time something set her off, I put her in a pelham.
I got one with a gentle mouthpiece, one that I knew she already enjoyed, and worked with her in it.
It turned out that after I displayed my new "magical" power a few times, the bolting disappeared completely. I would ride, as normal, on the snaffle rein but have the curb rein in my hands, ready to use, if the opportunity presented itself. As soon as she'd try to take off, I would just crank on that curb rein and shut her down immediately. Since I was not relying on the curb rein for more than just a few shut-downs, she came to the conclusion that I just now have this "power" indefinitely. Now, there's no question of her bolting because, even though I've switched her back to a snaffle, she still thinks I have curb power. I do sometimes put her in the pelham again, just for a little reminder ride, but it's extremely rare that I actually have to use the curb rein. I think I initially used it all of twice before she figured out that I was on to her game.
However, in her case she fully knew that what she was doing was wrong...she just knew I had no power to stop her.
In your case, I would focus on just walking out on the trails and get him fully under control at all gaits in the arena (if you can't reliably bring him down a gait in the arena, imho, he's out of control) THEN, and only then, try doing more on the trails.
I thiiink what I found with my girl (she had very similar issues initially) was that vocal cues have a lot more power with her than physical ones. When she's out there running and I've just let her go for it, I could pull on the reins all day and she would not care. However, if I say "LACEY! Ahh-ahh-ah! Easy!" (plus, sometimes, a mild slap on the neck to get her listening to me if she's really into it), she'll slow up, even with zero rein cues. It's something to try. :)
I would try to stick with the bit you're using and just school him more more more. I don't think trying a single jointed snaffle would hurt but don't fall down the "little harsher, little harsher, little harsher" pit.
Good luck! It's hard! These competitive horses! Haha
Fabio - 13 year old Arab/QH gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.
Last edited by Wallaby; 12-17-2012 at 02:13 AM.