First, there is no way you can tell from the video that the face is exactly square at impact.

Start with getting him to strengthen his grip. To the point where he says, "What?!?! This position...no way!"

When he hooks them, start to weaken it little by little till it's straight.

Have him make sure his grip is light. Start to pull the club out of his hands where is just starts to slip out...then swing.

Have him relax...especially the shoulders. Allow the club to release with a snap and be sure this is not forced with the arms but the hands as the last move.

__________________
I'm a golfaholic, no question about that. Counseling wouldn't help me. They'd have to put me in prison, and then I'd talk the warden into building a hole or two and teach him how to play. ~Lee Trevino