holy shit that is a painful looking squat.
What type of backsquat are you trying to teach this guy? Because the position he is in would have to be low-bar, in which case the easy answer is to simply have him widen his stance considerably and arch his back more.
However if you are trying to teach him a deep high-bar squat, he needs to learn how to break at the knees and hips simultaneously from lockout, and sit DOWN rather than back. Think bringing the inside of your hips to touch your heels… Keeping the weight on the back half of the foot driving up.
Incidentally, the way a person squats with a heavy bar on the their back, and the way they squat with no weight at all added is going to be different because its an entirely different center of gravity. That is generally why people have their arms out in front of them when squatting with just their bodyweight (play the piano!), and not unnaturally behind their ears…
If you can work with the guy face-to-face, my advice is to get a sturdy box that comes up to mid-shin height, and tell him to stand over the box with it between his legs, with a comfortably wide footing, and simply squat down to grab it with his hands (hands hanging down at all times) and stand up with it.
This should give him the idea of sitting DOWN in order to drive UP. The added benefit of the box is, he can stop in the hole and stretch out the bottom position as much as he needs to, since he probably has tight inflexible hips as everyone does…
Once it looks solid and he is used to the movement it shouldn’t be too hard to repeat that form with a barbell on his back.
hm… thats all i got i guess… i was sort of obsessed with squat form for a while. go figure.