As a general guide, if a horse has his head up in the air, consider a curb, such as a kimblewick or pelham. If the horse leans on the bit, consider a gag type of bit or a waterford mouthpiece.
Keeping with Myler bits as to be fair i cannot fault them..
What about going for a Mullen Triple Barrel with hooks so you have some leverage?
Either that or go for a combination bit.
You could always phone the Myler company to ask their opinion, they're brilliant
If you can ride with 2 reins, use a 3-ring gag or a pelham. Pelhams are really not cruel if you use both reins.
That way, you can ride him on a gentle snaffle with no leverage, then pick up the contact on the bottom rein when you can't control him.
If you can't do 2 reins, maybe learn. Whatever you do, do not use roundings, because then there is no point in 2 reins. It is better to use one gag rein on a gag than a pelham/gag with roundings.
If you always use a strong bit, he will become used to it and there will be almost no improvement in his behaviour.
I'm another fan of the Myler combination bit; I used to ride mine in a three ring gag on the last ring; but now I use the combination bit, and its fab!,
He's much more relaxed and I have good brakes.
Myler Combination with double reins? Worked fine for mine, snaffle most of the time and the curb rein when extra communication was required.
Do you have a martingale on him? No bit on this earth is going to stop him if he has his head up to a point where the bit is acting on the corners of his mouth rather than where it should be?
Also, just a thought, have you tried him in a hackamore? I had a mare that was a complete lunatic with a bit in her mouth (any bit) yet she was a doll in the hackamore, even out hunting.
It is all a question of trial and error when everything else has been ruled out.
If his back and teeth are ok, instead of using a stronger bit, I would advise trying a stronger noseband.
I hunt my two in french-link Bauchers (hanging cheek snaffles), and use a drop noseband for Bubbles who gets a bit 'pully' towards the end and a Kineton for Mo who pulls as soon as we get on grass!
Hope this helps