MY suggestion is to drop the weight to something he can handle in proper form even if that is only the bar. then slowly move the weight up as he learns the form.
I would also say a box that is a tad below parallel would be a great choice. That way if he is falling back he will land on the bench. It will also teach him how to sit back and imrpove strenght out of the hole.
Stretching his hip fl;exors,. hams and glutes may be a good idea as well.
Hope that helps,