There is no way I would let a pitching coach make that big of a change without understanding the why? I recommend that your son talk with coach and ask him why? Tell your son to share with coach about arm pain. Speak with your son after they talk. If you are not happy with reasoning behind change, ask for meeting via email or phone. Be prepared with specific questions and your plan, should you not agree. There are some great coaches out there who make these decisions based on mechanics and what is best for your son's short and long-term development. There are others who only care about wins and will do whatever it takes to get those wins. Then you have the coaches who just don't have a clue. Have seen them all at some point.