When I bought my OTTB I had gotten her rubber covered bits. they're very good for sensitive mouths, but they are able to do their jobs. My OTTB was very energetic and didn't like going down from the canter, for me personally, rubber bits are my life saver.
What bit was he ridden in before? If it was my horse I would have teeth re-done and back checked before changing bits. Sounds like he is not happy with bits at all, even mild ones like you have already tried...safety aspect of hackamores are good, they are severe bits and only need a light touch.
How about no bit? Have you tried a english hackamore (without the curb insert) or there's another bitless bridle out there that supposed to be really good for stearing etc...sorry I can't remember the name of it!