Sounds like he is being a brat. Many horses aren't fans of going backwards because they can see directly behind them. To soften him up ask him to go backwards gently. If he doesn't want to MAKE him go backwards. As soon as he takes two steps release all pressure!! Let him walk forward, stop and back again. You want him to connect the gentle cue with "asking" and the not so gentle cue as a correction. Just as if you were going to ask a horse to move forward, you use a light cue/voice. If there is no response you move to something firmer, like a kick or a crop. Give them the opportunity to listen the first time, if not make them sorry they didn't listen the second time.
DON'T demand a lot either. If he backs one foot with light contact, GOOD JOB! Just keep building on it. In a few days one step will be two. In a few weeks two steps will be 5.
Also, work backing into you regular routine. Trot around, stop, back a step. Lope, stop back a step. Circle, walk, back. Don't just pick a specific time to focus on it, make it just another thing you practice.
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