If the student is leading his turns with his hips he is not over pivoting rather over rotating, and with low edge angle is causing the tails to wash. This will create a "pivot point" around the ski tips. I believe pivoting occurs when the femurs rotate in the hip sockets and it sounds as though he does not have this ability? He needs to learn to turn his feet independently of his hips.
The motion of leading the turn with the hips will also create a tendency to move the pressure to his inside/uphill ski and tip the shoulders into the hill. Perhaps work on balancing early onto the outside ski. Demonstrate with a traverse how balancing on the outside ski requires the hips to move inside while the head remains over the outside/downhill foot, then take it into turning.