Shallow Squats?

[CarnalSalvation]

A stretch that helped me get to a deeper box (although, keep in mind, I wasn't this inflexible) was to sit on the box, and gradually change three things.
A. Move my feet out further
B. Point my toes more forward
C. Move my feet back toward my body

This really helped stretch out my hips and hams, and made squattting to a lower box much more natural and comfortable for me. Really, it stretches the whole posterior chain.

Get him to sit on the box however he can, and then get him to sloooowly work those three things, and make sure he keeps his knees "pushed out" while he's doing it.

 

[graedy]

I have very poor ankle flexibility. The good thing about that is, that i couldnt move my knees past my toes even if i wanted to.
But i think, the others pretty much said it all: the hip is the center of the movement, open you hips, push ypur knees out (not to extreme but...), spread the floor with you feet.