I agree that fixing this with this horse is advantageous, both to the horse and to you, as a trainer and horse owner for the future.
Being a drafty cross type, they can learn to use their weight and size to their advantage, to remove themselves from doing something they would rather not do. Once they find success with this, as you have found, you are pretty much at their mercy, powerless to stop them. The danger is that they may apply this theory to other things that you may ask of him, at some later point. So that is why it is advantageous to try to fix this, for him.
The key is to be able to tip his head towards you, when he goes to straighten himself and lunge away from you when he goes to bolt. You need a tool that will give you that power, something he can not argue with. The answer is a lunging cavesson of some sort, with the line coming off the top of his nose. A particularly effective type of tool for this is an iron halter. The nose piece is solid steel oval, which sits loosely on the horse’s nose UNLESS the horse goes to turn his head away from you, preliminary to bolting away from you. When that happens, the oval twists on his nose, and is uncomfortable for him. He immediately nixes doing this, and bends back towards you, where the pressure is immediately released. You only ask for a little bit of lunging, maybe only walk and jog at first. You set yourself up for success, and reward him for work well done. You don’t ask for a lot of lunging, only a few minutes, and stop before he gets tired, or does anything he shouldn’t. When he becomes more comfortable with cooperating with you, and accepts your appreciation of that, then you may try a canter. By then, he will hopefully understand that bolting is simply not something he can successfully consider, and you can reward him again for work successfully done.
It is hard to find an iron halter that is of a size that will fit a large headed horse. But you may be able to get one built for you, by someone with a forge. The only current source I have seen is a supply called “Dressage Extensions”, where the device is called a “Correction Halter”, or something like that. And in the picture, it is fitted upside down, with the hole for the line coming off the bottom, not the top. Don’t fit it like it is in the picture, turn it around so that the line comes off the bridge of the nose. But it only comes in one size, and since I own one, I can tell you it is not a very large size. It fits most normal sized TB type heads, but may be too small for a large drafty type or warmblood type head, because it is not supposed to fit tight. An iron halter is a very old tool, they have been around for a long time. In the wrong hands, with an emotional and angry handler, they can be quite severe. In educated hands, they are effective to teach the spoiled or ruined horse a specific lesson. Not many horses need to use one, or need to use it for longer than a few weeks, but if you have one of these horses, the tool is a good one.
Good luck!
Poor lunging is mindless circles. Proper lunging is, at the very least, excellent mental training - teaching a horse to think before reacting, using it to get his focus (back) on you when he’s upset or “high”, and more. From there it can be incredibly useful physical training.