Deductions & credits

You, as the Custodial Parent, are the only one that can claim your son every year, and file as Head of Household for the Earned Income Credit. 

His father can claim him only as a Dependent for the Child Tax Credit every other year, as per your divorce agreement, if you indicate that you agree to this in your return (screenshot).

You should consult an attorney if you want to change the terms of your Divorce Agreement.