You do not need to send the IRS anything, but you should print, fill out, and sign form 8332 (see below) and send it to the other parent. The other parent will send in the signed 8332 as well as an 8453 (which will help the IRS match it with his e-filed return)
The only other action on your part is to not claim your child as a dependent. Input the child's information, and then the software will prompt you with "do you have an agreement with the other parent?" This should result in an 8332 to be printed, but if not, print this one: https://www.irs.gov/pub/irs-pdf/f8332.pdf
Since the child lived with you for more than 6 months, you can still claim Head of Household (unless you remarried), the Child and Dependent Care Credit, and the Earned Income Tax Credit, if applicable. The other parent would get the Child Tax Credit.
