Like mentioned before, just take it slow, don't have an agenda, and don't push things. Leave the horse curious. Don't worry so much about him liking you, just allow him to become part of your world and the security that goes along with it.
I do put horses like this out with a buddy that is already quiet and friendly. However, I do not treat the abused horse any different than I do the other horse. Do not look at the exact actions, just at what can successfully be done without causing too much resistance. The biggest mistake that I see made with abused horses is that they are treated as abused horses, people tend to be very hesitant around them and that in itself will make the horse worried. Horses have an incredible memory, but they are also extremely forgiving. The horse, however, doesn't sit around and dwell on the fact that he was treated unfairly. The horse lives in the moment and deals with what life gives him. If we treat him like he is something that should have exceptions in his life, then he will always have them.
I don't put a time limit on a horse with emotional issues that they have to get through. They will give you everything in time as long as you don't try to get it too much. Horses are like little kids, the more you want them to do something, the more they will resist, the more you push them away, the more they will want to be part of your life. The abused horse has to earn its way into my herd just like any other horse. Any aggressive behavior from the abused horse is met the same way a dominant horse would react, they are not sheltered.
He should come around fairly quickly with consistency and fairness from you.