Originally Posted by sempre_cantando
Perhaps he doesn't really understand? You could try doing some ground work and ask him to back up from the ground with gentle pressure on the reins. If he ignores it, tap him on the chest with a crop until he takes at least one step backwards. Reward him, and practice this regularly. He will soon learn that gentle pressure on the reins is asking him to back up.
+1 - I've done this with two people. One ground and one saddle.