Thank you Paradismutts. What you can do is to teach Cody to walk backward on a flat floor, then "back up" on something like a stair or event a book. If he does not backup straight, put some chairs or any obstacle on each side and carry on. Once he got it, then teach him to back up on two or three stairs so that he gets used to climb a few steps back. Then put books against a wall and there we go. When he can back up on them, add books gradually. That's where we stand for the moment.
Once Fly has it, I will ask her to back up on the wall.