I have found that sometimes bitting up to a light shank bit can often be good for riders who are not so strong and riding lightly, and working back down to a loose rein on it, can actually be beneficial.
Had a little 80lb girl taking lessons on a hot horse, she couldn't control it in a snaffle, and while maybe I would of kept him in a snaffle and fought it out, she was not physically strong enough to. So, we bumped her up to one of these lovely billy allen bits: http://www.buytack.com/products/bits...lly_allen1.htm
After we did this, it gave this poor kid enough leverage that IF she got into trouble she could use it...If she didn't, she was to stay on a loose rein and pick up as light as possible. Remember you don't ride a shank bit with contact, you ride with a loose rein, and THAT is where people get confused.
Now, ideally, fix it in a snaffle. Get him softening to the bridle, lowering his head when you ask, giving both directions willingly. If the horse is soft, he will be less likely to brace and run through you.
After that I would spend a lot of time on a loose contact, saying "whoa", and if he doesn't stop...Back him off your hands. Say whoa, give him a split second to respond, then take a hold of him and back him up ten feet or so. Pause. Walk off again. Eventually he'll stop and try backing on his own. At this stage, move up to the trot. Then the canter. He'll get it, just be sure to be consistent. EVERY TIME you stop, BACK him up! Waste no time either.
Just remember that the shank bit is going to make a difference between this contact:
and this soft rein: