Let me explain why, THIS HITTER, needs to keep his front foot closed more than he is presently doing it from my experience.
He needs to be a bit more stable on the front side to have the rear hip rotate through where we both believe it needs to. So he is just not squishing the bug. Yes some hitters could land a bit open with their foot, but they still have enough strength to hit off that front side. He is unable to get the back side through because the back foot is the only thing helping him stand upright when he swings. I think both of us believe thats not good.
Once he begins to have the front side foot stabalized he will almost immediatley be able to perform the task of the hip action we would like to see.
Im sorry if I have seemed harsh. But I have dealt with this problem alot in instruction. There is certain verbage that just will get a Dad and son more frustrated and other verbage that will put them on the right path.
For a laugh, look at this video of what happens when someone tries to get into the proper hitting stance by attempting to do what they were told, verbatum. Or at least how they heard it and perceived it. Its after the bunt defense.
Proper Stance