It sounds like you already know what you are doing, but I'll post what I have always done to teach a horse to accept the bit. Maybe it will help with this horse.
First, the suggestion by Saddlebag to teach a horse to lower his head is a good one. Before you get to that point, though, you have to be able to get your hand on the horse's poll. I recently worked with a horse that would not let anyone touch his ears (and consequently the poll), so they were having a very tough time haltering and bridling the old gelding. I overcame this by getting a halter on him, then holding the lead, right at the snap with one hand and running my other hand up his forehead and over his ears repeatedly. Two keys in doing this: 1) He was not tied, just weight on the lead from my hand; 2) I did not try to rub his ears, just passed my hand over them. He would jerk his head away each time, and I held him under control with steady weight on the halter lead. I did this repeatedly without stopping, first on one side, then the other. After a few minutes he got tired of jerking his head when nothing bad was happening. Horses don't like to waste energy. When he decides jerking his head wastes energy and gives him no benefit, he will quit. It took me several sessions of 15-20 minutes each to get him to relax about his ears. He eventually decided he actually likes me rubbing his ears. I do it often, so he remembers that he likes it.
Now, stand on the left side of your horse, just aft of his head, you facing forward, and lay your right hand on his poll. Put down-pressure on his poll with your palm. Not a lot, just a little added weight. He will probably try several moves to get you to remove your hand. The very instant he drops his head (which is what you want), even a little bit, you instantly release the pressure and give him a good rub on the neck. Then do it again. It will take several times before he understands that you want him to lower his head when you put your hand on his poll, and that you will release the pressure when he does so. At that point you can start asking him to lower his head a little further each time before you release and reward. He will eventually lower his head as far as you wish.
Now, for bitting and bridling, hang the bridle from your right thumb, palm down. Place your forearm on the horse's poll/neck with your hand extending over his forehead. Drape the bridle over his nose, cheek pieces on either side with the bit hanging below his chin. Cradle the bit in your left hand (not the cheek pieces, but the bit itself) with the chin strap hanging out of the way (in position to slip under his chin when you slip the bit in his mouth). Now bring the bit up to the horse's lips by lifting with your right hand (the one on the poll - simply bend the wrist up to lift the bridle). Keep your right forearm on the neck/poll of the horse for control (remember you already taught him to lower his head this way). Now, you have the bit cradled in your left hand with the bit at the horse's lips. This brings your left thumb into perfect position to slip it in the left side of the horse's mouth, between his gums, right there where he/she has no teeth. Slip your thumb in his mouth and wiggle it against his tongue. He will open his mouth. When that happens, lift the bridle with your right wrist, gently pulling the bit into his mouth. Now, take your left hand, reach through the top of the bridle and pull his right ear through, then the left. Adjust the brow band and the throat latch, and you're good to go.
To unbridle, simply reverse the process. Slip your right thumb under the top of the bridle, placing your forearm on top of his neck for control (this discourages a horse from pulling away when he spits the bit out), place your left hand in front of his lips. Slip the top of the bridle over his ears and drop it down by bending the right wrist down, allowing the bit to gently slip out of his mouth into your waiting left hand, without allowing the bit to bang against his teeth. With a horse that tries to spit-and-run, I always hold the bit in place with my right (top) hand until he settles down, then drop the bit out. That's why the forearm on top of the neck is important for control.
I will try to take a few pictures and post them later this week to help explain it. Hope that helps.