Stand in front of your dog.  Step into him, placing your hands in front of him in a "pushing" motion.  As soon as the dog moves back even a step, vergally reward him with, "GOOD, GOOD, BACK."  This can easily be done in a hallway with the help of a wall to keep him backing straight.

If he is interested in a ball, you can also use the ball to focus him.  Move into your dog with the ball above and in front of the dog's head, moving the ball in a "pushing" motion as you repeat the command, "BACK."  If a food reward is used, it must be thrown to him to maintain your positions.  Eventually, use a hand signal only to back him, staying in a stationary position.