I'm surprised this was not included in your MSA (marital settlement agreement). The form has check boxes for things like uniforms, extracurricular activities, health care, and other costs as well as who gets to decide on the child's extracurricular activities, and where the child will be during things like winter or summer break, but I guess each state's standard form/template is different, if yours didn't have it. Why not try to contact your ex and see if he will agree to split the cost? Put it in writing -- text him or email him, in case he decides to backtrack. If he says yes, it will make things easier, because you can avoid the headache of lawyers, the back and forth of negotiations, and time in court. If not, or he fails to keep his word after putting in writing that he'd be willing to split the cost, you can hire a lawyer and have him/her draft a document that your ex and his lawyer can agree on, and go to court once you're all in agreement.