The people I bought him from used a grazing bit and when I rode them they said they just recently changed him to a hackamore Changing the bit on him so drastically from a grazing bit to a hack to a snaffle can play a big part in his responsiveness. Many horses are never ridden in a snaffle once they are moved up to a curb type and they will forget how to respond to one over time if they aren't ridden correctly or refreshed ever so often.
I think there may be some ill fitting tack This can play a huge part in it as well. Can you have either a saddle fitter or someone much more experienced with recognizing well/ill fitting saddles come out and check your stuff over?
When I ask for a one rein stop he just simply moves his whole body and spins like a maniac 100 mph! That tells me that he hasn't been ridden properly for much of his life. What I would do is start from the ground and work on some flexion exercises to try to loosen up his neck so that he will be willing to bend instead of just becoming a spinning board. Sometimes the flexion will come quickly and other times it will take a bit of time. I got a 12 year old horse a couple of years ago that sounds a lot like your guy. Very stiff, squishy brakes, no speed control. It didn't really take that long for him to re-learn what he should have already known.
I did ask a vet and she said he had a few sharp teeth in back so I am not sure how much that has to do with him NOT stopping That can have a lot to do with him not stopping. If he feels pain whenever you pick up on the reins, then bracing and continuing to go may be his way of dealing with it. He could be so focused on the "ouch" of the bit that he can't feel the cues that it's giving.
He also when I first bought him stood still to mount but just recently he has started moving away so I assume some kind of discomfort is there That could be caused by either a poorly fitting saddle or improper mounting. Do you use a mounting block? Are you balanced and quick when you mount? If the saddle is pinching him whenever you step on or you are unbalanced and really hang off the side and pull the saddle way off center, then that could be what is causing him to want to get away from that. Another thing to consider is what you are doing immediately after you get on. Do you immediately ask him to move off or does he just start on his own and you let him keep going? After you've dealt with any possible tack fit issues, then I would start working with him to make sure he stays standing until you decide you want him to move. Immediately after you get on, work on flexing him to each side at a standstill (after he is flexing from the ground, of course). If he walks off before you ask him to, back him up until he's soft and responsive in your hands, then let him stop and stand for a minute. Each time he moves off without your cue, then stop-back-sit again
He also chomps on the twisted bit I use I don't know why he does that either That tells me that either his teeth are really bothering him or that he is uncomfortable/unaccustomed to a snaffle or a twisted type bit. Chomping is not uncommon in a horse in his situation just because it feels completely different from what he's used to.
What I would do is get his teeth taken care of first, which you already said will be happening in two weeks, so that's good. Perhaps in the meantime, you can have someone to check your tack and make sure that it's fitting well and not pinching him.
Do you have access to a small enclosed space like a roundpen or even a small sized paddock? I would want to work him in there until I felt I had more control over him before trying to take him out in the world. That would also be a good place to try out bareback with a halter for the first time because it would be difficult for him to really take off with you in such a small area. Do you also have someone that could maybe help you try it for the first time? They could put another lead on him and get him stopped if he decided to act silly and try to take you for a spin.
Do you know what the grazing bit that the previous owner rode him in looks like? Perhaps, if you could find a snaffle with a similar looking mouth, he would be more comfortable in it. However, I am wondering if perhaps the previous owner started riding him in the hack due to his issues with the bit
. They probably didn't take the time to try to figure out what might be causing the problem so kudos for you trying to improve
on something instead of just ignoring and avoiding it
Personally, I would want to get him out of the twisted bit as quickly as possible, but you simply might not be able to for right now. After you've got him stopping well, I would advise you put him into a smooth bit of some sort. After the switch, you may have to re-school a bit more because it wouldn't have the same bite as the twisted, but the second re-schooling will go much easier if the first is done correctly.
Anyway, after you've ruled out pain issues, you'll need to start working on his training to get him responsive to the level you want. That is another place where a smaller pen would come in handy as most horses are less inclined to run if they realize there is nowhere to run to.
I would start at the walk, let him walk a few steps, then ask for the stop-first with your body (sit deep, perhaps put your feet just a bit forward at first), then with the bit. If he refuses to stop, then slowly increase the pressure on the bit until he does. The instant that his feet stop moving, release all pressure on his mouth and give him a good scratch on the neck. If he walks off before you are ready, do just like you would do when he walked off mounting and back him up a few steps until he's soft and repeat until he waits patiently for you to ask for "forward". Then, ask for the walk again and let him go a few steps before asking for the stop. Repeat this over and over and over.
It will take time and lots of repetition, but he will eventually be stopping whenever you shift your seat. Once he is consistently doing that, then you can move up to the next faster gait (on a stock horse I would say move up to the trot, but I'm unsure what the gaited equivalent is LOL). Once he is consistent and responsive at the faster gait, then you can move up to another faster gait.
Don't be afraid to do this on the trail either. I will often work my young horses in this fashion while going along a trail or on the way home. It helps to teach them cruise control and will reinforce the stop cue no matter which direction they are headed or what is going on around them.
After getting the stop down solid, then you can start working on his other issues under saddle like having to ride on a tight rein all the time.