A question for you guys - How do you desensitize with cows? Cows that are out in a big pasture and have a tendency to run away when you try to get close?
If he is going to do something as drastic as back into an electric fence, then you can bet your bippy I will kick, scream, yell, and whack with whatever I have in my hand to stop that happening. A horse that is so tuned out that he puts both of you in danger is a horse that needs his attention regained no matter how.
Personally, I DON'T want my horse to go up to scary things for a look. My preferred reaction is that they give me their attention, and continue on whatever line we were on and whatever speed we were going. I use my reins and my body to keep their head either straight or a little away from the scary thing, and ym outside leg to keep their body straight - And we just go on past. They realise that hey, they didn't die! They learn to look to you for direction.
I never want my horse to learn that the correct response when scared is to stop and look. That can be very dangerous, and when trying to compete or get a job done, very counterproductive.
OP - If my horse was already in the situation of backing up, I would go to something he knows through and through. So maybe yeilding his head around to my foot, maybe small circles, maybe sidepassing, etc. Anything to get his attention back on me, and not on evasion. If that didn't work, you bet that I would get that attention however I could - But the second he gives it to you, stop, relax, and go on.
You need to decide how you want your horse to react - It is a personal preferance. As I said, I want my horses to take confidence from me, and continue on the path and speed we were on, unless we need to detour. It works well for me, and my horses are generally very quiet and non-spooky on trails.