as stated the IRS will send notices to both asking to amend. if no one does, next comes a letter from IRS asking each to prove they are entitled. then the IRS makes a decision based on what is submitted. the one qualified keeps the dependent the other gets a bill. based on what you have provided, you should get the exemption. there is no way to prevent your ex from claiming even if he isn't entitled - happens 1000's of times each year.
