I always like to start on the ground first... sometimes for youg horses backing up can be a hard balance thing.
Walk next to your horse along a straight fence, stop and then face your horse directly infront of him, by respecting your space as you step towards your horse he should step away. (sometime you need a crop as insentive hold the rein with one hand and on the tap the horse on the flank with a dressage whip on the side not against the fence) The whip offers energy, your job is to channel that energy backwards- do not let your horse move forward or sideways.
Only take a couple steps, stop move forward and reward and try again. Be very concious of your body position during the whole thing. Another thing to note is backing is a 2 beat gate, like the trot diagonal legs should move together not each one by on it's own- which many horses do.
Once your horse is doing this well from the ground have a firend help you. One on the horse the other on the ground, have the rider incharge of the reins, remember it's half holts for each step not constant pulling, tip your hip's forward. Have the person on the ground still use their body language by facing the horse and using the whip as insentive. Once the horse relates your forward hip angel and half holts with backing your all set to back all day long!!
Just remember like anything in horses it's not going to happen over night, don't pound it into him just work on it a couple times each day you ride and reward every small improvement alot!! Good luck!