OP, try to figure out if there is something you can do that hits his reset on bolting. This takes time and a lot of observation.
If you can figure out when the bolt is coming, or maybe just assume that it’s always coming, you can start to figure out what will divert the bolt impulse to not happen after all.
There could be an internal horsey time limit before he starts becoming agitated. Or a particular environment or geography. Or a certain distance from the happy security place (a distance trigger is very common with barn & herd sour horses.).
Then it is what will get him to reset before he has the reaction. This can be tricky to suss, trial and error. There can be more than one trigger. Be patient and forgiving with yourself as you explore.
My non-tying horse (who did learn to tie, with safety qualifications) was spooky as hell, in addition. I gradually realized that these problems were coming from the same place.
It was like he had a constant anxiety ticker, that started on low, and gradually built up to high, until it came to the point where he had to have a release. Boom, pull back on halter and thrash, or under saddle have a giant spook.
I learned how to read the ticker, and what would get him to distract and reset the ticker time. As he got better and better, basically the ticker time was lengthening.
The reaction was coming. But the new habit was to finish what we are doing and move on, before the timed explosion. The horse came to expect this as well. That helped to gradually lengthen the time before explosion. Except that you couldn’t push the time limit by very much, at any given moment.
A Trainer said to me once during a ring lesson “just avoid those standards since that is where he spooks and we don’t have time to deal with it right now”. I replied, “he’s not spooking at the standards, he’s spooking because it is time to spook”.
The horse was going to spook at something, if there wasn’t a change in what we were doing that would hit the reset button. It worked the same with being tied. We worked up to standing tied for 20 minutes on a blocker tie, then we had to have a little walk as a reset. It really was not a big deal and was well worth it for this horse.
I learned how to read the countdown clock, and make a reset before the explosion came.
But this horse was still a special case all of his life, because as easy as it was for me to handle him (he became so easy after he learned to stand without being tied !! ) , other people just could not clue in to the magic formula, all in a minute. They had to relearned some of their default horse handling.
I had to do my best to teach the few people who sometimes groomed and rode him how to read and reset the countdown clock. And I had to recognize which people were just never going to clue in (a local vet tech, mostly!).