Stu, you do not want to shift your weight back at the leg lift. You want to close off your pivots (lift leg and shoulders) while you start immediately to lead with your front hip and drive your force vector towards the target. If you look at Trevor Bauer he does this really well.

His shoulders are dipping as a reaction to his front hip and force vector driving towards the target. This is very different than shifting your weight back. This hip drive is building explosive momentum. It sounds like your son had poor speed through his stride, so after front foot strike his shoulders where not level because he had very little forward momentum in his body.