My last horse Pride had some kind of history before I got him, to where he hated even the sound of a bit; even with a lot of desensitization he still seemed leary of the sound of the rings, and the chin strap...I got him to where he would take the bit nicely, but the fact that he was still seemingly afraid of it, bothered me. I was confused, because I am so careful especially when introducing a bit to a horse's mouth; So I bought him a rubber mouthed driving bit, which cut out the sound factor, and for a while, left the chin strap off, and attached it after I had the bridle on. After a couple of weeks, he was perfectly fine with accepting a bit in his mouth, and I was even able to go back to a regular snaffle (my normal bit of choice). I think someone, at one point had just shoved the bit in his mouth on a few occasions, causing both fear, and pain (the latter of which has the most lasting impression in a horse). I don't know it this helps you any, but especially if you don't know 'all' of the horse's history, and why he doesn't like the bit, it could be worth checking into; perhaps find a simple rubber mouthed bit (my driving bit was fairly cheap, just incase it didn't work, I didn't want to spend a ton on another bit!), and work at introducing it like as if he's never had one in his mouth before.
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."