you ex can grant you permission to claim you son by providing you with form 8332.
Note: If you are filing your return
electronically, you must file Form 8332 with
Form 8453, U.S. Individual Income Tax
Transmittal for an IRS e-file Return. See Form
8453 and its instructions for more details.
being behind in child support has nothing to do with federal rules for claiming a dependent.
maybe there's something in your divorce decree but the IRS doesn't care.