I would suggest going back to more basics again. It sounds like he has had a bad experience and remembers it. Load him in the trailer, unload him, load him, unload him and don't use the butt bar and don't shut the door for a few days, just work on these basics. Leave the trailer open and don't restrain his head either. Do this for several days. See if he'll stand there without any restraints until you ask him to back. Once he is to this point and he is taking the backing slower, use the butt bar/door, then open it up, let him stand there and then slowly back him up. Don't rush the process, use baby steps, let the positive backing times outweigh the bad experience and he will improve. When he's ready take him for a short ride around the block then back and see how he does.
Also, always untie his halter before opening the door, once you get to that point. Good luck!