Ill throw in one more, in my many many years of coaching wakesurfing (Skydiving, snowboarding, hockey etc) the biggest and most effective tool in my toolbox has always been DRYLAND TRAINING. get him to practice his body position and stance on the dock, the more time you spend there the better the result. Some people have a very hard time concentrating on tasks and tips while in/on the water, if he can practice on land you eliminate all the other factors that generally distract you from completing tasks. This is massively important in skydiving as you generally only get about 50 seconds per jump to work on skills or complete tasks (and flying thru the air at 100mph is so fun you forget your first name). Work on muscle memory, getting into the right positions, where your weight should be and what you need to think about next. repetition repetition repetition!!! i would suggest practicing his stack and mental progression in the boat each time before he gets in the water. This generally helps my most difficult clients the most. Good luck!