If you are the custodial parent, the easiest thing to do is file your tax return and claim your dependent. The duplicate claim will trigger an IRS investigation. If the IRS comes to you and says, he has a signed form 8332, you tell them it was forged and you never signed it.
If this has already happened and the IRS doesn't believe you, you may need an attorney, and to file a police report. You might want to do that anyway, but we aren't attorneys and can't give legal advice.
If you aren't filing a tax return to claim your children (which means there is no duplicate claim to be investigated) then you can file a separate fraud report.
https://www.irs.gov/individuals/how-do-you-report-suspected-tax-fraud-activity