Thanks for allowing me to assist you, I am the horse's servant and will do my best to help you with your horse. Sounds like your horse is angry and resentful. My first hunch is to really examine his saddle fit and make sure he is not being restricted or inhibited by an ill fitting saddle. As you know, a colt does not actually develop a "hard" mouth in a few days, however a mouth can convey resistance when a horse is not able to detect what is required of him. This is classic of a horse that worked well and was asked to do "something" more but the method of asking was not clear to him. The trainer/rider usually goes to something "louder" (bigger bit, bigger spurs, shorter tie down) and may indeed achieve what was desired but not because the horse understood what was required of him but rather through force and restraint. My suggestion is to go back to what he knows well and understands and start over. He clearly does not understand the idea of moving from behind or with impulse and speed. His flattening out and running away indicates discomfort, particularly of the saddle area. A horse has to round his back and tuck his pelvis underneath himself to gather for proper acceleration, but if a saddle is pinching or digging (common in many saddle/horse combinations), he will have to flatten out to protect himself. He did indeed move faster, but not in the way you desired. The solution will include slowing down to restore correct movement from behind under really well fitting tack so he can be comfortable and drop the resistance. A bigger bit is counter productive as it initiates more resistance in the horses body, thus slowing the training down literally and figuratively will help him "listen" to the bit when communication is more clear for him. I strongly recommend reading a book by Mark Rashid entitled "Horses Never Lie", as it will provide you with insight into your horses behavior. It sounds like he was going very well for you, and thus your relationship can likely be restored to a harmonious partnership once again. I commend you on seeking input from an outside source, it is a sure sign of willingness to aid your horse in being the best partner he can be.

As for his physical comfort, be sure his teeth aren't retaining any caps and have been seen to by a qualified dental practitioner; have his saddle fit evaluated by a qualified saddle fitter; check his physical condition with a qualified body work professional; and be sure his environment (stall/corral) are suitable, diet is in order for his age and size, and his hoof care is reflecting his current work load (as per timing of trim/shoeing be it 5 weeks, 6 weeks), and he is getting enough downtime with respect to turn out or grazing if available. A happy environment makes for a happier horse and it sounds as though you are willing to be sure your horse is good to go so he can work well for you.

I hope this helps, please ask more questions if needed, stay away from gadgets while fixing this problem, and listen to your horse. Good luck with him, he sounds like he will be a good one.