First of all does he understand what is being asked of him? Some need things broken down in to small baby steps. If he does and hes the lazy uber confident sort he may be a bit backward thinking and lacking respect.
If the latter he needs to be driven forward at speed any which way. If he's rearing he is not going forward. Loose schooling is best or if not just one line so you can keep him going forward whatever rein he chooses. Its important to stay parallel to his stifle area, any further forward and he will stop. Does he move away from the lunge whip? Ive resorted to attaching a carrier bag to the end of one before to get something moving.