As others said, get his teeth check first.
I also wanted to comment about using treats to get him to take the bit. We have a horse at our equestrian therapy center that is absolutely horrendous with the bit. His previous owners spoiled him completely, and he refuses to take the bit unless you have a treat with you. He resists to the point where he's dragging the pole/the person holding him backwards and looks like he's going to flip over. It depends on the horse, but I personally wouldn't take the chance to have a horse that will only bridle if you wave a treat in front of his face.
I also want to second having him lower his head. I would try putting pressure on his poll until he lowers his head with nothing or just a halter on. Get to the point where he lowers his head immediately when you put pressure on his poll.
One last thing, at my barn, we have a school horse that's impossible to bridle until you hook an arm over his ears. Then he'll lower his head immediately. Sometimes a horse might just have a little quirk.