If you want them to back up, you must halt yourself when they halt and step back with them. In the beginning, you can gently touch their front legs around the cannon bone with your boot (do NOT kick, think of your boot as a finger tapping someone's shoulder). Start with one or two steps and then release the "pressure" and praise him, so the horse knows he is doing what he is supposed to. As he progresses, you can take more steps and all you will need to do is walk backwards yourself. It also helps with backing up if your horse is sensitive to you when he is being lead around, as in when you slow down he slows down, you speed up he speeds up, you halt and he halts.