Q. So you agree when my decree states 'every year as allowed by law' means when she is NOT eligible to claim him based on IRS rules/law? (I assumed you meant not eligible).
A. No. I'm surprised you lawyer told you to see a CPA. It's a legal question, not a tax question. The tax law still allows her to claim him. What's changed for 2019 is that she now needs form 8332, from you, the new custodial parent. The IRS goes by physical custody, not legal custody*. "The custody plan/agreement expired in May at his HS graduation" is irrelevant
One other thing as changed: if she will be claiming him, she can no longer "claim everything", since she is not the custodial parent.
There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner.
Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child , even when the custodial parent has released the exemption to him.
* But, be advised that the issue of emancipation enters the picture when the child reaches emancipation age for his state. An emancipated person is not longer in anybody's "custody" and the ability to split the child is technically lost. In this case she would not "allowed by law". But, the father can continue to claim the child as a qualifying child, for tax purposes, because the child still resided with him (for more than half the year).
