He doesn't have a problem backing up. He is acting out his barn sourness. He is backing up as an evasion .
In any case, you want him to go forward and he's refusing and backing up to avoid it, right?
What you need to do is to disengage him when he starts to back up. So, you pick up one rein and lift it more or less straight up. He should band his head way around. (you must loosen the other rein so he can bend way around) you hold the rein , even if he is still backing, until he kind of curls around and he steps his hind end around. If he keeps backing in a circle, you lift the rein a bit more. You want to feel his front end come to a stop and his hind end step around that stopped front.
When he does, you turn his head toward the direction you want to go, give him a looser rein and "offer" him the chance to go forward. Encourage him. If he chooses to back, then go back into disengaging the hind end.