Your question has been answered: You can claim your child, for full tax benefits. You don't have to do anything special. Just file a tax return, claiming full benefits (dependent, child tax credit, EIC). The IRS rules favor you.
Your Ex is the one with the problem. If he doesn't like it, he'll have to file suit against you. The IRS will not help him. Worst case: the court finds you in contempt and sends you to jail. Most likely result: a modified court order.
