Horses are a flight animal. If they feel pain they run from it. This is usually the reason why horse's bolt. But, we need to know the cause of the pain. It could be in many places, but very often the pain is from the poor fit of the saddle.
Many people will say that their saddle fits. They have had the saddle fitter out to fit their saddle and because of this they feel that they have done all that needs to be done. But, the saddle fitter can only be sure that the saddle fitted on the day that they fitted it, because both the saddle and the horse can change shape. A fortnight later both could have changed shape, particularly if the saddle is newly stuffed and/or the horse has had a change in diet or exercise.
Use The Perfect Fit Saddle Fitting Kit now to check if the saddle fits. If it doesn't, be sure you get one that does and don't ride him in the old saddle. Using the kit, check the saddles's fit at least every month, or more often if there are changes to your horse's diet or exercise.
When you are certain that the saddle fits well, it could be that your horse is still bolting. This probably means that your horse remembers the pain that he was in previously, or, he is still in pain from the damage done by that saddle. If this is the case, you must have his back checked by an equine physiotherapist, chiropractor or osteopath. In fact, if a horse exhibits any unpleasant behaviour, the first thing to do is have him checked out by one of these professionals.
Sometimes it can take a while for a horse to trust that the saddle isn't hurting him, so be patient with him if the bolting doesn't stop immediately.