1) Get a copy of the 1095-A from your ex-husband.
2) If the 1095-A shows more than just your son being covered (such as both your ex-husband and your son), you need to ask your ex-husband if he will claim 100% of it (and you claim 0%). If he does NOT agree to that (or some other mutually agreed upon percentage), the it is divided by the number of covered people on the 1095-A (if it show 2 people, then each claims 50%).
3) Enter the 1095-A into the program to get it to produce Form 8962 (see link below). If your son is not the only one on the 1095-A, on the screen immediately after entering (or editing) your 1095-A, check the box saying the 1095-A covered somebody that is not on your tax return, and fill in the followup questions about the allocation percentages.
https://ttlc.intuit.com/questions/4348755-the-irs-is-requesting-form-8962-for-2018
