I believe in slowly, slowly with abused horses. It is very important that you gain their trust. If you have their trust you can work with them. Even though horses will never forget any cruelty done to them, I believe they can trust people that treat them well.
I am quite happy to spend several months working on a particular problem. I let the horse tell me what they are comfortable with and this dictates the speed at which we move through the problem. It's now been 2 months and I can hold both of his ears and move them back and forth. Bridling is no longer a problem and he is easy to catch.
I totally agree with the comments about putting him into a paddock. If a stable is where most of the horror happened he needs to be in a place where he will feel safe and with room to move.
The first thing I did was turn this head shy horse out into a nice paddock with my quiet dependable horse, who trusts me and will come up to me as soon as he sees me. I make sure that there is a bit of grass in the paddock but not enough to fill them up. They then need to depend on me for their feed.
I then got this new horse into a routine. I would feed him at the same time each day. I did nothing but feed him and talk to him for the first two weeks. After about a week he realised I was the food dispenser and he would look up expectantly when he saw me. Then I would add a few extra visits each day. I would go out a several times during the day with some pieces of carrot. My horse would come straight over and the new horse wasn't usually too far behind. No pressure. Just talk to him and give him a treat.
Eventually he was happy to be near me. As soon as I felt he was relaxed being around me then I would move my hand slowly toward him but only as close as he was happy with. The minute he looked worried I would retreat. Any time he moved toward me I would tell him what a wonderful boy he was and give him a bit of carrot. From this point forward he only gets a "good boy" and a treat if he moves toward me or let me touch his face. If he moves away I do nothing. Each day I could get a bit closer to his ears. You only need to spend a few minutes a few times a day on this – a maximum of 10 minutes a day.
This routine and moving toward the horse and retreating can go on for weeks. Sometimes you might feel they have gone backward a few steps but just keep steadily working at it. It takes a lot of time and patience but the feeling you get when you know that this horse trusts you is SO worth it. I also find that because you have worked through one problem and you have this horse's trust it will now be much easier to work through other problems.
Good on you for wanting to help this horse