Actually, what you're decree may say holds no weight with the IRS. The IRS has their own rules when it comes to dependents of divorced or separated parents. The only entity that can over ride those rules are a federal judge. Since federal judges don't deal with this issue, I'm 100% confident in stating such an over ride will never happen. Not ever.
As it stands for you in 2019, you have every legal right to claim your son as your dependent (based on the information you have provided) and this has absolutely nothing to do with any decree. If your ex claims him, let them. When the IRS sees that both of you have claimed him, you will both be audited (paper audit by mail only). The winner keeps the money, and the loser pays it back with interest, penalties and fines.
