Have fun with it, MAKE THE PICTURE CHANGE. Do it at 30% speed if you have to. You can't make changes swinging at full speed. Put a club across your shoulders and practice what Erik is doing at the 4 min mark in this video.
I think he understands this don't you @mackingchicken? We do say clarify it by calling Key #1 relatively steady. Basically it means we want to limit side to side movement of the head and allow for some vertical changes, especially on the downswing. There is definitely rotation. So the head isn't "still" meaning it's not locked in place.
Practice turns with a sock under each arm about and inch down from the armpit. . Try to keep the pressure under that location longer in the back swing as you make a turn. I am not saying SQUEEZE the living crap out of the sock. Just make sure there is a nice subtle pressure there to keep the sock in place. Just work on getting the turn earlier and quicker. The swing will feel like a TON of body and a slight raise with the arms/hands at the end.
I think that would be a decent drill for him, arms wouldn't be able to lift so he'd have to turn a little steeper.