At the present time, there are no Little League (or other baseball organization) rules about pitching. Young pitchers who have not completed their bone growth are especially at risk for elbow and shoulder injuries. Most coaches are aware of this problem but may not be aware of how much children are actually pitching.
Keep a pitch-count book on your child for a week (or longer). Keep track of the number of pitches in practice and in games. Record the kinds of pitches he's throwing. Keep track of how often he has pain, where it is located (elbow, shoulder), and how long it lasts after pitching. Show this to the coach along with the magazine article.
Becoming an educated parent and passing that knowledge on to coaches takes time. Check with the coach in a week and ask what he or she thought about the materials. Don't be discouraged if the coach doesn't seem interested in the information. Making the information available is the first and most important step.