I just read an article about pain and how horses remember pain and it can cause anxiety even after the pain is gone, they will anticipate pain. I think part of your plan needs to address the anticipation pain.
Perhaps you can start out using a soft rope that would be about the diameter of the bit your going to use. At first put the rope in his mouth and hold it there, for a second then remove it, let him realize something in his mouth is not going to hurt. Keep putting the rope in and out until he is relaxed about it, rub him and give him lots of praise. Then progress the amount of time he will hold the rope and maybe you can loop it over his head and lead him around using the halter, but with the rope in his mouth. Once he was accepting of the soft rope I would then progress to a bit and do the same thing, short little seconds of holding it and progressing. Once you get the bit on him leave it on him in a safe area and let him eat with it, drink with it and just get used to wearing it on his own so he can figure out it is not going to hurt him.
I would avoid riding him with the bit until he is comfortable just wearing it around, then start on the ground with very light pressure and release. That is how I introduce all my colts to wearing a bit, granted they have not experienced pain but I want them to accept the bit and be comfortable with it.
I do have one who experienced a very traumatic mouth injury, nothing to do with a bridle, at the age of 16, and this horse is broke, but I had almost start him over with bridling and wearing a bit. Two years latter now and he is still not the same as he was before the injury. But he accepts the bit again no problem.