If he backs up when haltered/leading I move with him until he stops, keeping contact or tension on the lead, and don't release until he walks forward.
Today we were walking on the road back to the barn when he spooked and ran backward. It happened so fast and I didn't have a tight grip on my 12' lead rope so it played out to about 9 feet. I just looked at him, said, what? And with a little feel he walked forward immediately. Luckily its a quiet country road so nothing to run into. He would have, I think, run into something had something been there. In those scenarios where I see him backing up I can plan to move him around by my own body position. In other words I move off to the side and he veers off in a semi circle.
In today's situation, I may possibly have been jerked off my feet. Would it have been better to hang on? Sorry for the dumb question. I'm here to learn.