Foot in the Dirt
Have your pitcher put his pitching arm-side foot against the rubber. Take your foot and drag a line in the dirt from the ball of that arm-side foot straight to the back point of the plate. Instruct the pitcher to land their glove-side foot on that line or up to 6 inches closed when they pitch the ball. Closed is on the third base side of the line for a right-handed pitcher and on the first base side of the line for a lefty. Keep an eye on that landing foot, reinforcing a good landing and correcting an open landing when you see it. By landing on the line or closed, the pitcher gets his hips and shoulders in a position to throw strikes.
Two-pound Medicine Ball
A young pitcher often tries to use the glove arm for momentum to increase velocity, jerking the elbow behind the back, which can send the shoulders off line and decrease control. Take a two-pound medicine ball and put it in the pitcher's glove. Have them pitch off the mound and watch as the weight forces the pitcher's glove-arm to stay stationary, keeping the shoulders on line throughout the delivery. Ask them if they feel the glove arm staying put, where it should be. Take the medicine ball away and instruct them to pitch while mentally maintaining that feel. Reintroduce the medicine ball whenever you see the glove arm jerking the shoulders off line again.
Tom House, a former Major League pitcher and an internationally recognized pitching guru, made the towel drill famous. The towel teaches young pitchers to follow through straight at the target to increase control. Simply twist up a hand towel and put one end of the towel in the pitcher's throwing hand as a baseball replacement. Set an upside-down bucket on the ground in front of the pitcher and instruct the pitcher to go through their delivery. But instead of throwing the ball, they'll hit the bucket by whipping the towel. Put the bucket at the end of their range of motion so that they have to reach out as far as they can to hit it, increasing their awareness of a straight follow through.
Build on the towel drill by making a visual-aid baseball. Draw a straight, thick, black line around the circumference of a leather baseball that is visible from 60 feet. Line your pitcher up on a straight line. You can use the foul line or the edged grass on the field as a guide. Instruct your little leaguer to hold the ball so the line runs parallel to the fingers. Have them deliver the ball to you with the feel of the towel drill in mind. Watch the ball carefully. If you can see the line rotating straight up and down, they've successfully delivered a controllable and repeatable straight pitch.