He pivots beautifully on the forehand but I am having a hell of a time getting the back to plant and his shoulders to move. Any tips?
I understand he needs to learn how to do this. I am working with him from the ground as well as from the saddle. I am asking for tips to teach him. Not for the reason he isn’t doing it…

To get him to turn on his haunches or hind end, ride him down the fence and when you get to the end (corner) turn him into the fence (not away from it). This will make him use his hind end to make the turn. Use your outside leg when turning him into the fence ( horses move away from pressure). I use this technique a lot in my younger horses as well as my barrel racing horses to get them to use their hind end more. It helps a lot.