Dressage Legal bit for hard mouthed horse

First a brief history, my twelve year old QH was used as a western speed/rodeo horse before I got him. His past owners rode him in a twisted wire snaffle because he had "a tendency to run off." When I got him (three years ago now) I used a pelham bit on him. Worked really well, never had any running off issues, but his mouth is very hard.

Flash forward to about half a year ago, gave up on making him a jumper, and started working on dressage. Problem is, at intro and training level, pelhams are not allowed. Have tried him in o-ring/egg-butt/d-ring snaffles with no luck, half the time I might as well not have a bit in his mouth.

I do suppling and bending excercises until I am blue in the face, but he doesn't become any more responsive.

Any ideas on a good bit to try? I really am open to anything legal for my level.

If he has been in a pelham, have you tried a unjointed mullen mouth? Very simular to a pelham but has no shank or curb chain.

If he rode in a pelham which is a leverage bit, I don't think a mullen mouth would be strong enough. I would def. Go for a jointed bit... try a full cheek maybe.

Which is not a leverage bit either.

NO leverage bits are allowed in the lower levels of dressage.

My Quarab could not go in a regular jointed bit. Just the way his mouth was formed. He did however really like the pelham which I could NOT use in the lower levels. Since the pelham I used was made very much like the mullen mouth I showed in the mullen mouth very successfuly in the lower levels until he hit the higher levels.

So in my experience I have found it to be a good subsitute bit and legal.

I know a full cheek is not a leverage...and that leverage bits are not permitted for Dressage. I was just saying that because he has a stronger mouth, maybe a full cheek would work better because it also applies pressure to the lips and outside of mouth....

IMO he needs more foundation put on him. A can get a soft mouth again, it'll just take time and patience, but it can be done.

Mullen mouth and a pelham are not similar at all. A pelham allows for 2 sets of reins, one at the mouth and one on the shank, which works by applying pressure to the poll, which is what the bit is known/ used for.
Mullen mouth refers to the mouthpeice of a bit. It is not a bit in itself. You can get mullen mouth loose rings, mullen mouth d rings, mullen mouth pelhams...
It mearly means that the mouthpiece is made from a single piece of material which has a slight curve to it for tounge relief (but not as obvious as a ported mouth). Suggesting a 'unjointed mullen mouth' is not necissary as mullen mouth is unjointed.
Spyder by your reasoning, one could argue that a loose ring snaffle is also very similar to a pelham...

I second Winstons suggestion, maybe with a thinner than average mouthpiece. And lots of half halts.

Exactly and acts on the mouth in an entirely different way than a jointed bit does. All pelhams I have ever used had the same curved mouth piece (I refuse to use a ported pelham, or a jointed one) and while I know there are different mouth pieces available I go by what I know from my experience. If the pelham used by the OP is different than what I believe, they can come and say so.

Single jointed mouthpieces tend to have a nutcracker effect and if the OP got this horse having been in a twisted wire snaffle, I would, if I were training the horse move away from any single jointed bit.

The basic problem with this horse is the holes in its training as Spirithorse stated and that is where the real emphasis should be, but I would attempt to repair the training under a bit that had a different effect on the jaw of the horse.

You made a statement that a pelham is very similar to a mullen mouth. That is false, and to someone who does not know any better, misleading.

Just because YOU only ever use a mullen mouth pelham, it does not make a pelham the same as every other bit with a mullen mouthpiece. Suggesting that it is so, without offering any explaination of your reasoning, is irresponsable.
A pelham is still very different regardless of mouthpiece, and works in a very different way to a snaffle, which is what I am assuming you meant by mullen mouth.

Ok well, back on topic.
No leverage bits are allowed in lower level dressage. Any sane person using a pelham is going to be using a mullen mouth pelham. If you are using a jointed or ported pelham then you're going to have a bigger problem on your hands as far as scaling back your bitting just because something with leverage AND pallate pressure is a very severe bit.
My advice would be to work the horse on the lunge in sidereins and a regular french link or double jointed snaffle just to let him get used to a bit again. Hell I'd even ride him in a bitless or halter just to get that bad feeling out of his mouth. Then I'd pick up working him in a really soft bit. It's going to have the same effectiveness, as long as you don't overuse it. If you have really soft hands and don't ever pull, then when you do give an aid with the bit, the contrast between a soft hand and an aiding hand is still going to give him an aid, but this time it's just instead of the contrast between a pulling hand and a pulling harder hand.
If taking time off to rehabilitate the horse isn't possible, or if you are just looking for something to chuck on him for a dressage show then I would look at Baucher or b-ring snaffles. I know they are legal in Canada, not sure about the states, but they are the harshest thing allowed in lower levels.
Sprenger Baucher Bit Baucher Bit, Argentan Bahr Saddlery for all your equestrian needs Tack Shop Saddlery Store for Equestrians Horses Equine Products Dressage Hunter Jumper Eventing Supplies Apparel Tack Ontario Canada
WIth the baucher, it just gives the illusion of slight poll pressure to the horse. Just don't use it upside down, because that's illegal too. It's right side up in the picture.
Like spyder said, if he is used to working in a mullen mouth, then don't chnge it up, just find a baucher mullen mouth. However if he is used to a ported or jointed mouth piece then I'd try to find soemthing with either a french link or single joint. Keep as much of the bit the same as possible.

Good luck!