If you are the custodial parent, meaning that your son is in your household over 183 days of the year, then your ex does not give you a Form 8332. You are the one who provides the Form 8332 to your ex so he can claim the child's personal exemption.
You just claim the child on your tax return as a dependent. If the IRS ever questions your claim then they will ask for proof that you were the one eligible to claim the exemption. The IRS notice will give you the types of proof required.
