The non-custodial parent CANNOT claim the child without a written release from the custodial parent. The custodial parent can voluntarily release their claim by filling out IRS Form 8332 or equivalent, and providing it to the non-custodial parent. The non-custodial parent then must submit the form with their tax return.
If your divorce decree went into effect after 1984 and before 2009, and it contained wording specifying 2016 as a year in which you had to give up the claim, he could submit a copy of the decree instead of the Form 8332. If the divorce went into effect in 2009 or since then, he'd need the Form 8332 or equivalent from you. Hope this makes it clear.
Again, if this continues to be disputed, just submit your tax return by mail, claiming the child. The IRS will settle it.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
