I've been dealing with a rescue horse, too. I had been trying the standard natural horsemanship methods, such as Parelli, to try and get him gentled. He didn't want to be brushed either. But he wasn't scared. Actually, he became rather aggressive and dominate once he settled in. I've been looking into Carolyn Resnick's Methods recently and she does everything at liberty in the beginning, unlike Parelli. I've done the water hole ritual of "sharing territory" and he responded like never before.
For grooming, I was following the advance/retreat and he was not getting much better. I then followed Resnick's suggestion of: if you approach a horse, you respect his space and follow his lead; if the horse approaches you, he respects your space and follows your lead. When I want to groom him or when I go into his space, I wait for him to look at me, take a step back, then go on in. Wait for a "yes' on the grooming, retreating when he says "no", then retry. All done at "liberty" meaning no halter/rope and he is free to go at any time. He responded the best with this approach. I can now groom him, pick his feet, give meds, etc. without resistance and at liberty.
She also says it's best for you to wait for the horse to come to you for haltering as this means you are the lead and he is following you. If you do go into his space, you must show respect for his space, he will respect you for it. She has a
blog and YouTube channel. I've just ordered her book Naked Liberty and her DVD, hoping that I can understand the waterhole rituals before I try any more of them.
I also found a blog that gives some detail on the
rituals. The "sharing territory" is the first one and that's the one I've done with great results. Also, I basically used the "taking territory" for his food aggression and it's working. When I first got him, he gradually became more and more aggressive at feed time to the point where he bit me. I keep him from the food(in a non-aggressive, non-punitive way, just wave him back) until he looks at me with both eyes and forward ears - a good expression. He still wants to be demanding, but backs away easily and it takes less and less time for him to look pleasant.
I'm very impressed with this approach. I think it would be especially good for fearful horses as there is no pressure or punishment, it shows the horse you respect his space yet expects respect from him. It establishes you as the leader.
Good luck!