If you have to keep the weight lighter, in order to get the lifter to learn to keep the pushing muscles balanced and keep the firing in proper order, for him to learn this,( and thus his muscles and CNS will learn the proper pattern in even pushing) then stay lighter with good form and slowly move the weight up over the months to cement that form.
Otherwise, not seeing his posture or if he ever sustained a shoulder injury etc., thinking that this will just correct itself with heavier work, to me is probably not the best approach.
I liken it to when the knees come in during a concentric squat. You just have to become aware you are doing it, and stop it. Making yourself push upwards evenly.
If you cannot do the rep properly without odd twist compensation, then stop the rep and learn the proper path way and firing order of the muscles.
Flexibility and stretching may be in order too.
ZirRed may post some good PT stuff!?