The back position is indeed proprioceptive, and is discussed on pp37-38 of BBT. I hope you can help him before he kills himself.
The eccentric portion of the squat is essentially resisted extension, not active flexion, which would be a catastrophic wreck.
The barbell certs are 19 hours of instruction in both fixing the lifts of the attendees and teaching them why we did it that way. There is a lot of material.