Ex spouse needlessly using the court system & costing me a LOT of money.

It sounds like you and your ex- might be in a bit of a power struggle or at least a pattern of some sort. If that is the case (it may not be), then the best answer might be to try to break the pattern. Since you are in a 50-50 parenting plan arrangement, you would likely agree that your child needs his parents to be as effective as co-parents as they can be. That too does not appear to be the case.

As a proactive step, you may wish to consider co-parenting counseling with a therapist who is experienced and skilled in that area. (Many are not.) It may be an ounce of prevention that will not only pay off for you economically, but also pay off for your child and for yourself.

Assuming that the mediation process is listed in your parenting plan, then engaging in the process in bad faith would be a basis for fees. There may be other bases as well -- best to consult with an attorney about your circumstances. Ultimately, however, you are likely best off if you and your spouse can learn to become parenting partners instead of a parenting adversaries.