Take him to the easiest terrain wherever you ski. Go to one edge of the slope. Have him stand on his uphill edges. Have him step uphill on his uphill edges. Point the skis in a shallow traverse in the uphill edges and have him ride until the skis turn enough to stop him. Point the skis in a slightly steeper traverse and do the same. Try from gradually steeper and steeper traverses, riding the uphill edges to a stop each time. To vary the "practices", have him step uphill toward the end of the ride to get a stop.
Now have him put a finger into the front of one boot right at the very front. Tell him to "crush your finger" by bending his ankle to pinch the digit between his shin and the cuff of the boot. Have him remove the finger but "crush" it again and again with ankle flexing. Then have him do one of the steeper traverses and try "crushing" the downhill finger. By now he should be able to ride the edges out of the fall line to one side and stop more quickly with a "crush".
Don't take him to any slope where he has to use a defensive braking wedge to control his speed. Introduce him to gradually steepening terrain only if he can continue to use his edges to turn and stop.