Thanks again everyone.
Oceanne, I must confess to using the occasional food incentive to get my horse perkier about doing his job, or to clarify exactly which behavior I'm trying to isolate, but the moment I even get a hint that he is focusing more on the treat than doing as he is told, the treats go away.
ChrisDoctor I started teaching him to back that way with the halter and lead on. I would stand at his side, behind his shoulders(like where you would sit if riding) and cue him very gently with the lead, to prevent him from turning and facing me I would redirect his nose straight ahead with the stick when he bend instead of backing. Then I started cuing from his hip and then from behind incorporating a gentle pull on his tail into my back up cue. When he was doing that well(2-3 lessons) I started using the tail pull first and if he didn't respond then I would reinforce with the lead. Soon he would back off just the tail pull, so then it is a matter of using fewer and fewer tail hairs in the cue, until you need none. Now he very clearly makes eye contact and "asks" if I want him to back, I back up myself if my answer is yes.