I would stick with a milder bit and go back to his basics. You need to re-train him how to properly use his muscles, specially those in his neck and over his back. Doing a lot of transitions in the walk and trot will aid in being able to control the canter later on. But it's hard to control it if his head is up like it is in the pics.
Teaching him to "keep his head down" will not have the same effect as actually training him to carry himself in a balanced relaxed frame. And you can come to need the training aids and they can't help you in the ring.
You horse must first be able to go long and low before collecting.
You do not want to use a twisted snaffle. That is going to do nothing to help you or your horse. The answer is not in a harsher bit which is what a twisted snaffle is. Just work on your transitions and working on moving your horse in working his back end into his front end. That only is what is going to allow him to move properly throughout his whole body and rounding properly. Rounding is not just in the head and neck. It is in his back his legs and bringing his butt up under himself and working properly. There is no quick fix nor is it going to happen overnight. Keep working with him in transitions serpentines, circles, bending, trotting poles. But do not switch his bit it will do you no good.
If he is used to western riding, thought you say he wasn't trained for it, I am going to assume he is used to being ridden in a shank. The western horse I am riding english right now is ridden in a Kimberwick port bit, but they also make Kimberwicks in the snaffle style as well. If he is more responsive to western bits, being a western horse previously, he may respond to a Kimberwick because they apply the same type of leverage as a shank bit does.
Just my two cents, may not work for him. It is working great for the QH I am riding now. Oh and don't worry about him being a QH and trying to show hunters. These are the two horses I am going to start showing in the hunter ring:
Beamer-Arabian x Saddlebred aka National Show Horse