If he doesn't back from the ground, I wouldn't ride him (personally). He needs to b able to go, stop, turn left and right, and back from the ground (in long lines) before he should be ridden. If he has forgotten, there's no better time to re-teach him! If you aren't sure how to teach him to long rein (which can be done anywhere except your dining room), just ask. It will help him figure out how to lunge when you teach him that, too.
A backing horse is one of the most frustrating things to deal with, I think. Remember he's a horse, and horses don't spend their time figuring ways to make us mad. We are people who are prone to give in to impatience. Think of the positive outcome, and you will notice yourself become more patient.
EDIT: I read your post wrong. If he doesn't do it on the ground, then back him from the saddle. Either way, you need control over his butt. From the ground will get the basic point across, so if he does it mounted, a few feet backing vigorously should remind him how tired he got the last time you made him work.
I have said what I have said. I have not said what I have not said.
Last edited by RunSlideStop; 06-30-2012 at 12:20 AM.