AYou might also try squirting ACV, Heinz brand, not imitation, in mouth as that will tenderize the tissues. 10 cc is what I would use.
Short of nerve damage, mucous membrane tissue(the bars of the mouth) remains sensitive regardless of pressure - it's incapable of callousing. Therefore it doesn't need 'tenderising' - strong bit pressure still hurts a 'hard mouthed' horse - what is 'hard' is it's mind, not it's mouth & it's learned to brace & ignore it.
OP, I agree with those who have suggested a hackamore or bitless. I've had a fair bit of experience 'retraining' 'hardmouthed' horses in this manner. *Hopefully if he was well trained & you're skilled, it won't take him long to get him back to where he was, but I believe the more you try to force him, the more you're reinforcing his current *attitude* & resulting braciness. Hopefully well trained & skilled also means you both understand seat & leg cues well & I'd make the most of that too.
If going the hack, I'd use a real one or a rope halter, not a leverage device. The point is to *teach*(in this case, reteach) the horse to *respond* to gentle pressure, rather than attempt to force him to comply. The reason I think it's best to do this without a bit is that he's already got his habitual attitude & braciness to the bit pain. I would avoid a bit for the time being until he's going well & reliably in a halter or such.
I wouldn't worry about finding something strong enough to stop him bolting back to the stable - you probably won't find anything that in his current head space will work anyway, short perhaps of some severe torture device. So I'd firstly keep him in a safe environment, where you're able to *ask* for stuff but there's no need to attempt to force it.
I'd start out on the ground, teaching or reinforcing him yielding in different ways to halter pressure first, especially as depending on the type of halter/bitless you use, your rein signals will have a different feel. Then I'd do the same on board, starting with just standing there & getting him yielding his head around, or 2 reins means back up. *Yielding is not forcing, but teaching the horse to give with a light feel. If that's stating the obvious to you, great - I don't know what you know, so rather give too much than not enough.
You take that feel, perhaps increase it to a level of *mild* discomfort if the horse braces(it depends I reckon), but then you just hold that pressure & wait. Don't increase further or try to hurt the horse, just wait. Eventually(might take a while if he's previously learned that ignorring it makes it eventually go away) he will move in such a way as to relieve the pressure from himself. At that instant, release all pressure & you can also positively reinforce that instant, to strengthen the message further. Might take a number of patient repetitions of this to start sinking in, but I've found that esp with well trained ones that have been spoilt, it doesn't take much to remind them what good deals feel like. Once the ball starts rolling, I find it does carry over & everything becomes quicker to learn, but it's still important to go at the horse's pace.
Then you do the same at a walk, including slowing & stopping to *gentle* pressure, getting good & reliable with that before trotting, etc. I'd want to have him reliable at all paces before 'testing' it by taking him out. I'd also start outside in the same manner - have him master a walk before asking for more.