Depending on the specific facts and circumstances of your case, there are any number of things you can do. Your ex may be in violation of an existing order, in which case you could ask that he be held in contempt of court. You could ask for a change in custody from joint to primary, or you could simply ask that the court admonish him to stop his inappropriate behavior. More details are needed. You should consult with an attorney experienced in child custody issues.
It does not sound like you have any basis for a TPO. Behavior is very difficult to monitor. If the decree required him to share gymnastics expenses and he does not, you can file for contempt if he stops. If he is not required by order, it is unlikely a court will make him pay. You should at least speak with a family law attorney to know whether you have any recourse, and what that might be.