You and your Ex must execute IRS form 8332 which you can download from the IRS document site
on the internet. Get two forms, one for your exemption and one for his. In such form you should indicate that you will be alternating each year the right to claim the deduction for the child. Whether you are the custodial parent or not will not matter.
If you need further help, please call me at 212-370-1660.
Leonard M. Weiner, Esq.\Divorce Solutions