he probably just spooked and his natural extinct kicked in and he fleed
This sounds like the symptom of a bigger problem. I recommend you lead him with a long rope and hav him go in and out of the field with it.
This means you are not in control and not because you are not dominant but because your horse doesn't consider you as a safe place to be so first thing that happens he runs off to protect himself.
Thank you. I will try that. We have done a lot of ground work recently because his whole back is dodgy and getting sorted by the vet but I always look at him to make sure he isn't hurting so I will try not looking at him.
Oh, and to add, if he does end up trying to get away from you and you find yourself being dragged along, lean back with all your weight, like sitting in a chair, just with no chair! Get low to the ground, and keep your arm that's holding the lead rope as close to your body as you can, and keep it locked tight. This puts your center of gravity lower to the ground, and more "in one spot" and it makes it much more difficult for your horse to drag you around!!
If I'm reading correctly, when you say you can normally handle him, it sounds like he does it regularly? Is he generally a nervous animal or does he only do it when you lead him to the pasture? If it is only when you lead him to the pasture, take time to school him on it. If it has turned into a regular thing and your horse isn't really the nervous type, it may have turned into a habit, which is disrespectful and dangerous to you. To start, try putting your horse about a foot and a half behind you, and off to the side a bit. If anything happens and he tries to gallop off, he will go by you, and you won't end up getting plowed over. If he tries to walk around on your other side, swing the end of your lead rope in a circle towards him. If he gets hit with the end of the rope, that's fine. It's teaching him boundaries, and that he can't go in that space, so he has to stay where he is, in the correct spot. Once you have this down, start walking. take about 5 or 6 steps, and stop. When you first start out doing this, Say "Whoa" out loud, and slow your steps a bit, so he knows that something is about to be asked of him. If he walks by you, stop him, and back him up 4 or 5 steps, then repeat the process again. When he does it correctly, you can feel free to give him a treat, if you wish. Make sure to do all kinds of different things, Walk in a straight line, walk in a different direction, stop a lot. Another important thing to do is to NOT LOOK BEHIND YOU. Don't look at your horse while you're walking because it puts him in the dominant, or leading position. Just walk confidently without looking at him, and it will encourage him to look to you, and trust where you're going. Eventually, you can put your horse back up next to you, and not have problems. I did this with both my mom's and my horse, with alot of success. I hope this works for you too!