Once when I was bridling my mare, I was inadvertently bitten because I had put my thumb in her mouth to help open. Without realizing it, I started bridling too fast (puling the headstall up too fast) after that and finally, when bridling, my mare protested by sticking her nose in the air and refusing the bit. At the time, I didn't realize it had been developing for the last few rides and thought it was a sudden thing. At my next lesson, my trainer advised me I had been pulling her headstall up to fast and banging her teeth, unconsciously trying to avoid being bitten. The trainer's remedy was to back up my mare every time she put her head up to avoid the bit. Then she walked her forward and tried again. It took about 15 minutes of this until she was bridled. Then she was unbridled, and the trainer tried again, taking several tries until she was quietly bridled. When my mare began accepting the bit quietly, it was my turn. I did have to back her up a couple of times, but it went more quickly and I was able to bridle again normally. I have ever since been very careful about bridling. It is really easy to get into am bad habit and not realize it.
If you can correct him in a similar way and see if he is just being difficult about bridling because he can, it is possible it is not his teeth. If he still gives you problem, then yeah, maybe his teeth.