8 year olds mechanics

My sons pitching coach tells him that he is opening up to early. What can he do to help eliminate this. I am assuming this means when his plant foot hits the ground his shoulders are already around. Am I correct on this? Thanks guys

There are a number of possible causes for early shoulder rotation. The most likely causes are posture issues (e.g. leaning back or leaning the glove side shoulder towards home plate) and glove issues (e.g. dropping the glove, pulling the glove or flying open with the glove).