If you are not your son's legal custodian, it may not be possible for you to instigate legal proceedings to prevent a move of schools - it may be better to try and reason with your son's father, and ensure that the situation is created by wanting the best for your son, rather than an attempt to upset you personally. Calm discussion is far more likely to yield results than attempted legal action. You are going to be your son's parents long after you have both moved on personally, so it's important to make sure his interests are served best - by both of you.