Your refund will not be reduced for your fiancee's child support arrears. The federal Refund Offset Program can be used to take the refund when a married couple files a joint return, and one of the spouses has a debt subject to offset.
Whether you can claim him as a dependent is a different subject. You may be able to claim his exemption, but he could not have had income for the year of $4050 or more. See this link for more information about claiming an exemption for a girlfriend or boyfriend: http://blog.turbotax.intuit.com/tax-deductions-and-credits-2/can-i-claim-my-girlfriend-or-boyfriend-...
But whether you claim him or not, your refund won't be applied to his child support arrears.
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