Under the special rules for children of divorced or separated parents, only the parent where the child lives more than half the nights of the year can use the child as a qualifying person for head of household status. If this is your ex's year to claim the child as a dependent, you must give your ex a signed copy of form 8332, which you can find under "other tax situations" or download separately from the IRS web site. Your ex can claim the dependent exemption and the child tax credit (except your child has aged out of that) but the claim for HOH and also claims for certain tuition benefits always stay with the custodial parent.
See here
Publication 501, Exemptions, Standard Deduction, and Filing - IRS.gov
