How can I counter horse constantly backing?

Horse backs up. Won't forward longe. Backs from Vet. Backs from trailer. Backs when tied. Seems like a Conditioned Response. He rides fine in the Saddle. Tried direct pole pressure, puts head down but no forward movement. I've held the lead taut and chopped on it with hand. He backs. He just won't drive from behind. Seems like he wants to hide his and face things head on while backing away. He's big man on campus, so he doesn't seem afraid. He's so eager to please, I really doubt it's stubborness or rebellion. He leads pretty good with a rope halter, but will still back up. We've been building up trust, but I'd appreciate advice on friendly forward ground cues. Tapping irritates him. Twirling backs him. Etc.