If you have been ignoring the problem for months or years, you should not expect that the court is going to hold her in contempt the very first time you raise the issue with the court.
If your ex- argues that your son doesn't want to see you, but has not filed a motion to modify parenting time, and the court for some reason lets that argument continue, you can point out that the motion before the court is to enforce the existing order and that there is no motion before the court to modify visitation, and then ask the court to enforce the order. If mom wants to modify the order, she can bring her own motion -- but unless and until the court modifies its order she is expected to obey the order.
If she's represented and you're not, you should give serious thought to hiring a lawyer, lest you find yourself out of your depths in court. It's easy for an unrepresented parent to get steamrolled, even in cases in which they should have had the upper hand.

