Way 1 - Ask the horse to pick its hoof up. If it does, reward with a treat and place it back down gently. However, if it does not pick its hoof up, squeeze his tendon and he will usually pick up his hoof.
Way 2 - Dig your elbow into the back of the horses knee so they reflex by lifting their hoof. Reward with a treat if they lift their hoof. Eventually lessen the pressure you put on his knee so eventually when you ask him to lift his hoof as he should, he will willingly pick up his hoof.
Always reward with a treat when retraining. Consistency is your best friend here!
Work with pressure and release to avoid nippiness, however. When he lifts his hoof willingly, put it back down and continue to do this, so he learns that you will release his hoof when you pick it up.
Hope all goes good! :-)
Try setting up some ground poles. If that doesn't work use a log or a 4 x 4 so they don't want to hit it.