Is your child the ONLY one on the healthcare policy, or are there other people (such as your ex)? If there are other people, your ex SHOULD be claiming 100% of the 1095-A on his tax return.
If your ex does not do that, or if your child is the only one on the 1095-A, I think the only solution is to contact your divorce lawyer (unless your ex is willing to give you the money for the repayment). It should be pretty clear-cut, because if your ex is responsible for insurance, that means he is responsible for PAYING for the insurance, not you.
When your ex had filled out the healthcare application, if your son is the only one on the policy, your ex should NOT have entered any income information, because your ex is not claiming the child as a dependent.
