Just something that I found works with my horse when he wants to be a butt about backing-- when I am leading him (try this with a halter first, hold the lead close to the halter), I will frequently make an abrupt stop and say "ho" to let him know he is to stop with me. Then I will ask him to "back" while pulling the lead towards the center of his chest, asking him to tuck his nose and, if necessary, leaning my arm/shoulder on his chest to give him the idea that I want him to move away from my weight and back with me. You can use your weight to keep him backing.
You can do this any time you are leading him around, just "ho, back" and get him used to responding to your verbal cue. Then, when you're on his back and you ask for the "ho, back," simultaneously asking him to flex at the poll in the same way he would on the ground, he might get the idea a little better. :)
This is just something I have done-- not sure if other people would corroborate it!