You are going to need to sue him. You can file a Motion for Enforcement or a Breach of Contract... it's going to depend on the exact language of the decree. Sit down with a family lawyer to discuss which option works best. Good luck.
This answer is intended for informational and educational purposes only and should not be considered legal advice nor forming the attorney client relationship. This attorney is licensed in Texas.
You need to meet with a family lawyer immediately to discuss filing a Motion to Enforce against your ex.
Do not wait; make an appointment today!
If you wait too long to enforce the divorce decree, the tools available to your lawyer will decrease over time.