If I may ask a question. What is his ball position in regards to his stance? The reason I ask this is that he so many things right, and he has an in-square-in club head path per your video. If his ball position is just foreward of the "square" portion of his swing, and in the "in" part of his swing, might his club head be cutting across the ball just enough to cause the slice spin? Maybe moving the ball back just a little might help.

If I am butting in, my apologies. GJS