Ground driving is the best way to practicing with a bit in a horse's mouth with the same feel of someone pulling the reins on their back. Try doing some lateral flexion both while you are on the horse, and before you get on. You could try doing exersises that teach that he has to give to pressure. Also, you might need to be a bit harder on a horse that's first starting out. It goes with giving to pressure. The pressure has to start out soft, then get harder.
I hope this helps a little bit. Pretty much as long as you don't try to lunge the crap out of him like most people do, then you should have a great chance at having a him trained well. Giving to pressure is a main key to a having a respectful horse.