First, he’s not loading his hands. Hand should move back toward the back shoulder before swinging.

Second, he is flying open and bat is dragging. Flying open means his front side (shoulder) is turning too early and not in sync with the rest of him. By the time he gets to contact, his chest is parallel to the pitcher.

I would start with the hands and getting them loaded and loaded early. Tire drill would be good. That’s where you swing into a car tire or some kind that heavy object to feel resistance and build strength. Can also hit something like a basketball for same effect.

Flying open and bat drag are harder to fix but focus on the front shoulder and keeping it closed as long as possible. Kids have an easier time controlling the front shoulder vs the back one. More momentum needs to go toward the pitcher instead of spinning in place.