It sounds like he is trying to protect himself and sees you as predator. No trust. There was a bunch of articles in a H&R or Equus magazine a few years ago about a filly that I think acted the same way — rescued mustang. Every article was an update on the progress — or lack thereof. This is not something that is gonna go away overnight, unfortunately. You mentioned you do have experience with mustangs so you know what is involved. The calms may help. If nothing else, and you have tried for a long time, you may have to lay the horse down just to try to get his attention but this would be a last resort. Continue to be the “leader,” treat him like you would a “normal” horse (ie not easy just cause he was abused), watch your back, and keep working with him. If he never comes around, he would not be safe for anyone. That happens too.