I'm seeing that opinions are fairly varied with regard to which issue is causing the rest. Introducing separation, losing the stem, getting forward, turn shape, and edging have all been mentioned. To me, it seems like some of these things are dependent on others. For example, I mentioned getting forward, because it is difficult to initiate a good turn from the back seat.
I think I would agree with nateteachski that turn shape might be where his best chances for a breakthrough may be.
As Josh Matta pointed out, he is using his upper body to turn instead of using his skis.
I don't see how he can even begin to work on something like separation until his turns are happening at his feet and not his shoulders. At this stage, if you were to tell him to keep his shoulders and pelvis facing down the fall line, he wouldn't be able to turn.
I might suggest working on pure carved turns on very low slopes to start. J turns seem to work pretty good in my experience. Let him get a feel and understanding of how the ski bends and carves an arc. Without that, he has no choice but to slide the skis around. I guess you could say he is forcing his turns around with is body because he doesn't know how to use the skis to make it happen.
I might also suggest working on edge control and side slipping. I think it is just as important for a skier to understand how to release an edge as it is to grip with an edge. A perfect carve uses a firm edge grip. A brushed turn uses a controlled slip. Everything in between should make nice S shaped turns that originate from the skis.
As for losing the stem, I'm not all that concerned. I view the stem as a safety blanket. It's what you do when you are scared. You see it a lot when skiers are trying steep slopes for the first time. You see it at the beginning of the turn because that is where the leap of faith has to happen, where you have to face downhill with nothing to hold you back. I would certainly agree that relying on a stem to start your turns isn't good skiing, but I think it will fade away if the skier becomes more confident in their ability to use the ski to turn and trust their edges.
Thats the way I see it. Again, I'm not an expert on the topic, but this makes sense to me.