Divorce decree ordered alternating years on taxes.
Ex has not paid support in over a year, since my son turned 21.
My son has lived with me full time and commutes to college where he is a full time student.
Both parents pay 1/2 tuition. Ex has son on his insurance.
The IRS does not enforce local court orders. If you provided over half of his support for 2018 and he otherwise qualifies to be claimed as a dependent, you can claim him.
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.