refusing to walk..., story of my life. Flicka does the same thing when she wants to be done working. Quite annoying, isn't it? The trick is to give use his bad behavior to your advantage - when he backs up, make him keep backing up. Don't get mad at him - when he starts backing, just keep asking him to back and back for 10+ steps; he'll surely get tired if it soon. After several steps stop asking for the back and see if he'll walk on without complaining. If he's still fussy, ask him to back again. It might take a few tries at first, but soon he'll figure out that he just has to deal with a little more work. It will turn into a bad habit if you let him get away with it too many times.
Oh, and remember to praise him when he walks on so he knows he's doing the right thing!
partners in crime since October 2010