What a pretty fella!
First order of business is to eliminate any and all physical reasons for the horse to resist. Your horse isn't crossing/locking his jaw for his health, and the grackle isn't doing anything but masking a symptom. How old is the horse? Could he have teeth coming in? I would schedule a dental exam first thing if he hasn't been seen recently. Look into his saddle fit as well as any potential muscle/joint pain that could be contributing to his resistance. Your horse is trying to tell you something.
What kind of bit do you normally ride him in? You said it's a happy mouth with the grackle setup, but they make happy mouths in a lot of styles, snaffles, gags, curbs, etc.
My "fix" to your problem will be dressage/flatwork based, so if that isn't "your discipline", please bear with me a bit.
First, reevaluate your tack. You've got a lot of hardware in the pic you posted - is it all necessary? I would take it back to a well-fitting saddle and a snaffle bridle with a cavesson fitted only tight enough to not be dangling. I recommend a French link snaffle, adjusted only tight enough to maybe create 1 wrinkle at the corner of his mouth. The French link will minimize any nutcracker effect and mouth discomfort. Lose the martingale for now - it won't be necessary for some basic flatwork exercises.
Second, you say that he gets excited and resistant at the canter - I'm betting that he does this at the walk and trot, too, but in a less extreme way. Go back to the walk and try some basic suppling exercises.
Start with a loop in your reins, riding the buckle. Ask for a turn from your seat and legs. Sit tall and "kneel" in the tack, look in the direction that you want to go, apply your inside leg at the girth (think of that leg as a fencepost that the horse is curling around) and your outside leg behind the girth to encourage bend. Feel his ribcage swinging with each stride and use that to your advantage. Lastly, open the inside rein to guide his nose where it needs to go. (This is an exercise described and demonstrated in much greater detail in this video: Riding Basics | Horse Videos – Horse.com Video Library
of these videos are worth watching
). When he's calm, relaxed, and accepting of the exercise at the walk, try the trot.
Here's some more on just allowing the horse to relax and work freely without getting chargy at any gait: Riding Basics | Horse Videos – Horse.com Video Library
Practice whoa and go on a looped rein. This video explains these ideas much better than I can:
You may need to reevaluate your use of your hands and your position. Address any reasons why you may be contributing to his resistance through your position or use of your aids. If he starts even thinking about locking up and bracing at the walk and trot, start bending him "Chris Irwin Style" as outlined above to supple him again. Don't think about pulling him up, think about easing that tension and resistance. Pulling just creates more pulling.
Sorry for the novel. Perhaps anyone with more experience dealing with ingrained resistance like this can add to or correct my advice.
More on Crossed/Locked Jaws: http://www.horse-sense.org/archives/2001046.php http://www.stabletostable.com/forums...se-locking-jaw