If you were eligible to claim your child as a dependent in the past seven years, and you didn't use Form 8332 then it is conceivable that your ex wife's current partner may have been eligible to claim your son as Head of Household. It is common to alternate from year to year between parents when both are eligible. Here is a TurboTax article that explains the rules for claiming a dependent.
The IRS won't tell you if someone else claimed your child, unless your tax return is rejected for that reason.
Form 14039 would serve to say that your identity (social security number) has been stolen. If your SSN was used by someone else to claim the child, then this would be appropriate.
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