The reason why he opens up so early is because when he leads with his hips, he pushes off of his back leg diagonally collapsing the back knee moving it into early rotation. By creating early knee rotation, the front foot opens up therefore leaving velocity on the table.
It all basically starts with your back leg. You have to use ground force correctly to move your body more efficiently (balanced) down the mound. You also have to learn how to torque your hips as you start leg lift. Torquing the hips will also help you lead with the hips properly riding the body down the mound. Pushing off diagonally will only cause your knee to bend in and create early rotation causing you to open up early. This will also cause your your upper body to tilt to far back in your stride. Also, while in your delivery, concentrate on showing part of the bottom of your foot to the catcher driving down the mound. This will also help you remain closed longer. Take a look at the pics below and you can see what I am talking about