So you're saying that the head flipping and inverting are worse when you apply direct rein pressure?
If that's the case, I think there's lots you can do to help, but it's a long retraining process.
I suspect two things; first, that your horse was never taught to give correctly to rein pressure, and two, he's in the habit of traveling hollow, and doesn't have the muscle along his topline that would allow him to travel correctly with head and neck stretched down and forward. The first photo shows the lack of correct muscling pretty clearly.
I would start on the ground with a plain snaffle. And by that I mean no shanks/no leverage - a Tom Thumb is really a curb with a broken mouthpiece, not a snaffle. A snaffle with a double joint, since you say that's what he prefers. Work first on having him give to the side with pressure/release on one rein, then pull/release or flutter both reins. What you want to happen is for him to drop his head and flex at the poll; what will likely happen is he'll invert and fling his head around. Keep working, quietly and calmly, with small amounts of rein pressure, always matching the release to the amount of pressure, until applying pressure on both reins causes him to drop his head and flex, rather than invert and fling. Praise lavishly and repeat.
Repeat these exercises after grooming and before mounting. Under tack, do a lot of walk/halt/walk, walk/halt/back transitions. Any time you get a give to rein pressure, rather than resistance/inverting/flipping, praise lavishly.
Be prepared for the horse to revert to the old behavior when he's "up" or stressed.
The long term fix for this involves lots of lunging and lots of slow flat work to teach him how to carry himself correctly and build muscle along his topline. Search this site or any other good horse site for "building topline" and you'll get lots of info.
Last edited by maura; 04-10-2012 at 08:02 PM.