That video kinda sucks, but I looked at the one before it. He's got a little looseness in the bottom and tendency to lift the chest a bit. That will hit the knees. So watch for him descending too upright, or lifting the chest early (knees slide forward either way).
First thing I'd do is have him slow down a bit on the descent. Not super slow, just a little more controlled going into the hole, so that he can really stay uncomfortably tight the whole way. As recommended earlier, TUBOW can help feel this.
Probably get his toes out a little more than they were in the last video. His stance looked good in video 2, from what I could tell.