I just wanted to give you some encouragement. My first horse ever pulled back so hard the first day I owned him that he broke two lead lines and almost sat on a car that was parked near him. Not a great start for a novice horse person. Actually, his favorite direction was back (and still is really). I mean for everything, fly spray, water hoses, etc etc etc. He had (has) terrible claustrophobic issues.
I did extensive ground work with him and worked with a very capable friend in addition to a great trainer. I didn't really focus too much on the pulling back issue but worked on all aspects of respect and giving to pressure. He is now fine to leave tied to a trailer even in strange situations and also I have personally witnessed him get to the end of his rope and step forward when he feels the pressure.
In the trailer I use the blocker tie. (And for some time I used it outside the trailer.) Mostly because I feel that if they loose their footing, the blocker tie will prevent them from hanging themselves. If your guy stops fairly fast when he is scared then he would be a great candidate for the blocker tie. I basically gives a bit without completely letting go so the horse does not feel "trapped".
It is a scary thing, I know to have a horse pull back. But I think that if you are patient and work on all levels of giving to pressure, it will get better and maybe even resolve completely.
