A twisted wire O-ring would work lots better than a smooth mouth O-ring if he is truly hard-mouthed, and at his age at this point, with the kind of riding he's dealt with, he may very well be!
Even if you are inexperienced, as long as you are gentle and ride in safe, enclosed area, like an arena that's not too big, or a larger round pen, you can put to use a martingale, specifically a running martingale, I don't too much care for what they call a training fork, it ends up putting the rings too far back towards the horse if not adjusted correctly, and those German martingales just truly befuddle me, and are WAY too constrictive on an already stressed out, high-headed horse!
A simple, twisted wire O-ring will give you a little bite too it, make sure it's copper-covered so your horse can "taste" it and the twisted wire part gives it more "feel" than the smooth-mouth ones, especially useful for a horse that's already tuning you out, anyway, using split reins, NOT roping reins, it's NOT a good idea to use roping reins to "cheat" having to hold such long reins properly, again, like using the German martingale, on a horse like this just sounds like it would be too much for him to bear, so use the regular Western-style "running" martingale, adjust the buckle for the rings to be about halfway between the horse's mouth and their chest, the weight of the rings on the martingale, and the relief of the O-ring work well to "remind" the horse to keep their heads down, wherever your hands put them,
For instance, keeping your hands level with the horn, in front, on top of the withers, teaches the horse a normal headset, nothing too extreme, and the horse punishes himself by raising his head too high, and bumps the bars of his mouth with the bit, I promise you he will bring it back down REAL quick, they don't like the pressure, you can ride him around to let him get used to this going on, just stay at a walk for a good long while, ride him in circles, pretty large ones at first, nothing too tight, remember, you're not trying to get him limber, you're working on his headset! Also, try backing him, expecting him to keep his head level with the horn, but DON'T "hold" his down for this, just stay gentle and consistent, and he'll do it all on his own, keep your hands level and he'll get the point rather quickly, just get used to the "lack of control" that riding in the O-ring will leave you feeling, the martingale just kind of throws in that extra control, once he's been in this long enough, he'll probably need to be transisted to a twisted wire, sweet six, or twisted wire jr. cowhorse bit, with or without nosebands.
It sounds like you have a nice horse on your hands, he just needs some more training, and he'll be super-nice!