Have his teeth checked and maybe use a running martingale on him to keep the reins from flipping up.  It may also help you to keep him from popping his head up to rear. 

 If it wasn't happening before he went to the first trainer, I would say there was some serious ill fitting equipment or mishandeling.

In addition to having the teeth and mouth checked, I would also consider another area.  Have his back checked.  It could very possibly be out of line and this is causing pain, discomfort.  I have had similar problem and it was horses back.  I sent my gelding to a chiropractor (sp?) and horse is fine now.  Actually this was a QH but I also had same problem with a TWH that needed a spinal adjustment.  The cause? Well, I have this like for tall horses despite my being only 5'4 and when I would mount, I would have to pretty much pull myself up, pulling on the saddle, which inturn pulled the horses' backs out.  Lesson learned, jump higher and quicker when mounting or use a bucket! Lol!

Good luck!

I have a horse that did the same thing and found changing to a saddle that fits properly and taking him to a chiropractor has helped tremendous.

I can tell now when he needs a treatment he'll start flipping his head & reins.  I usually try getting away with doing a spring treatment and a fall treatment before going on a hard riding vacation.  I did have to do three last year but it's worth not having the behaviour problems you have to deal with when a horse is sore.

If it showed up at a trainer then I'd blame the trainer for the issue and tell people about it.  I don't know if you feel it's worth following the second trainer...I feel you have some hesitation about them also.

I had a pony that did this because once he crow hopped I'd cry, get off and put him back out in the pasture.  A GREAT lesson to teach a horse, misbehave and get your way.  We hired a teenage boy that came out and broke him in about 10 minutes with a dozen eggs...follow me and I know this sounds crazy.

Step one get on horse with a pocket of eggs.  Once mounted grab an egg.  When the horse starts to elevate to rear as his head comes up smash an egg on his pole.  Same thing next time, but I've NEVER seen a next time.  It goes off the theory that the horse will think there is a roof over his head and the egg simulates him bleeding but with no pain.  We used the other eleven eggs.

A more aggressive method which seems to take a longer amount of time to really work and is more teach with pain( I hate these methods but some do have a place with dangerous horses) is to use a bat or crop to pop his pole upon rearing and smacking his shoulder when head tossing.

My approach would be changing his bit.  If the first trainer was heavy handed (and a good chunk of WP trainers are) and this caused the initial behavior the horse has associated the bit with imminent punishment and is fighting back before it happens.  Bring the horse home and change the bit and I bet you'll get your WP horse back.