My son just turned 15. His pitching coach suggested he stretch his arm using rubber bands or a towel. How is this exercise done?

Buy some surgical tubing bands varying in different kinds of resistance strength, this is what I do. Then do shoulder exercises with them. The reason a player has a good arm is because he has a strong shoulder. Do this and play long toss every other day, and you will get where you want to be if you are committed to the program