@Reeves Martin agree but you can also see at foot strike all his weight shifts into his toe which could also cause the back leg collapsing because it’s just an unbalanced position
@Reeves Martin incorporate some med ball work to work on balance at foot strike and ploys like roll ins to get comfortable with moving your body and learning how to balance better at foot strike
An effective change up doesn’t have to be necessarily hard. It’s a matter of Seam orientation that works off your fastball. One change up grip that helped me a lot was a kick change. Tread has a really good thread on twitter that you should check out