When teaching a horse to back up, really make sure that you make yourself as big as possible. Make as much noise as possible, make yourself physically determined to back him up (I'd do this in private, cause you kinda start looking a bit crazy when doing this
). If possible, use the end of your lead as another intimidation technique.
While you're doing this, take it slow... Very slow. Start off with 1-2 steps back, and then reward him, whether that be with treats, or a break, or food, or a quick rub. Then very gradually make your way up to longer stretches of steps.
Just be intimidating and take it slowly :)