But when were the LAST time his teeth were done? They NEED to be done every year...and I prefer an actual dentist because their schooling on teeth is longer than a vet's.
That depends on what state you are in. In my state it is illegal for anyone other than a licensed vet to float or otherwise treat a horses teeth. Because of this, many vets in my area take extra CE on horse dental care. Personally, if I couldn't trust my vet to be well trained enough to take care of my horses teeth, I wouldn't trust them to do much else on my horse and would find a new vet.
OP, I learned to jump on an appy who sounds similar to your horse. She would refuse to open her mouth when I would try to bridle her. My trainer would allow me to try for around 10-15 minutes, then send someone out to help me. Every time the more experienced person could get it right in her mouth and it frustrated me so much, because it seemed like they were using the same trick of sticking my finger on her tongue that I had been trying unsuccessfully for so long! Eventually I could get the bit in her mouth in a short amount of time, but it took me months.
The horse was smart, like Appys tend to be, and knew the difference between me who was new to her tricks and someone who had experience with her who she knew she couldn't pull anything over on. Eventually I became one of those people too, but it took a lot of time and work on my part to prove that I wasn't going to put up with her crap.
If you have ruled out all physical options, then I'd say it is probably the same with your horse. He is smart and knows he can push you around. I would get a trainer of some sort to work with you, so that you can show him that you won't put up with his antics anymore.