"we made some swing changes to mostly get him to use his lower half"
He's not using his 'lower half' at all. His stride movement is not good.
MLB hitters coil their shoulders and hips inward as they go forward.
It looks like someone tried to take away his natural stride and now he toe taps, and coils OUT as he moves forward. When you coil out like this, you begin the swing before the toe touches the ground. At toe touch, notice that his back elbow has already dropped and his rear forearm is already mostly vertical (not good), versus horizontal (good); plus, his front hip is open (not good).
Watch Posey here (0:02-0:07) how he coils in, then at toe touch (0:08) where his rear forearm is.
To fix, simply take dry swings and coil hips and shoulders in and hold until you land. That's it, it's that simple, and it's free. In real swings, you begin to uncoil just before you land, but you don't need to practice this, it happens naturally.
Also, make sure that he begins his stride near when the pitcher takes the ball out of his glove. It seems like he may be waiting until he thinks the pitch is a strike to start moving. Try holding the ball in the cage and ensure that he moves on every throw.
Take videos from the catcher view like this to see how he is progressing (also try to get the pitcher in view so you can see when he starts the stride).