I've had this problem. Constantly remind the student that he must move the rudder control in the direction he wants the aircraft to go, ie nose right - right rudder. Lots of practice taxying at increasing speeds. However, even when the student can taxy using the controls in the correct sense, he will tend to revert to trike controls when pre-occupied with the round-out, flare and landing. Keep your feet off, but very close to, the rudder pedals until he can demonstrate proper control. Good luck!