Instead of relying on a bit, have you tried a bitless bridle, side-pull or non-mechanical hackamore on this horse? Before riding, start with Clinton Anderson and/or Parelli methods to teach him to give to pressure using ground exercises first (this disengages the hindquarters and provides most effective brakes). Many good instructional videos are available on their websites and YouTube. For instance, can you play “the friendly game” with him anywhere/anytime without him getting all excited (consists of gently tossing a long lead rope all over his body)?
Also very important IMO–make sure he gets all the turnout time possible with horse buddies (24/7 is best)! Years ago I had a friend who had a horse she called “DumDum” because he was so uptight and shied at everything. As soon as she moved to a barn where he was no longer confined to a stall, he rapidly became a completely different creature and CALM trail riding companion!
My final suggestion is that, based on his history, this horse is at high risk for ulcers. Chronic pain can often cause a horse to have a very tense and high energy personality; they may try to “run away” from it if they can (often described as being “very forward”). I recently discovered a mostly holistic and quite inexpensive method for treating horse ulcers and it worked wonders for an old mare of mine with Cushings.
Link is: http://www.spanishhorsetack.com/horse-ulcers-inexpensive-treatment/ Of course, have your vet take a look at it first to see if they might have any concerns. Best of luck!