If he isn't striking out much but most of his contact is a ground ball to the pull side, it is usually one of two things: he is coming up out of his swing, or his front shoulder is opening too early. Either one pulls the barrel up and puts him on top of the baseball.
The drill I use is front toss where the coach fakes the toss. If he is already out in front, he feels straight away how weak that is. Once he learns to stay back on that back leg, get his front foot down and drive the ball to the opposite side, he is using his whole body instead of just his upper half.
It frustrates players at first. That is the point, and it is where the results come from.