Nonpayment of Court Ordered Medical Expenses

My ex-husband and I have the same income level, so we do not have a court order to pay each other any child support. However our divorce decree states that we will split the medical insurance, medical bills, school fees and agreed upon sports expenses 50/50. I am the main insured adult on the insurance policy for the 3 boys, and due to that fact, I also receive all of the medical bills since I am the responsible party. My x husband owes me $912 from LAST year’s medical bills that he refuses to pay. That being said, I continue to pay all of the medical expenses in 2016 for insurance and medical bills so the total is growing. It’s not the amount he disagrees with, he doesn’t want to pay the entire amount because I was re-married in October of last year and he feels that since my new husband is very successful, that I now should pay for more than 50% of the children’s expenses. How do I force him to pay for his half of his own children’s expenses as they are outlined in the divorce decree? Keep in mind that my ex-husband earns well over $100K a year, so it’s not a lack of funds that keeps him from paying.