Under the Mandatory Victims Restitution Act of 1996 (MVRA), the federal government can garnish wages and tax refunds to recover money owed to the federal government or for unpaid restitution in criminal cases, even after the sentence is complete. See
U.S. v. Lee, 659 F.3d 619 (2011). That case describes the maximum amount that can be garnished (generally, up to 25% of a debtor's 'disposable earnings' for a week; further details in the case); see also
18 U.S.C. § 3613. If you have joint accounts with your wife, or file joint tax returns, there is a potential for garnishment of the joint asset. The reason you are getting a tax refund is irrelevant to their ability to garnish the refund - it's your money, not your son's. Presumably they are trying to get the money now because they have identified you as owing money and having the ability to repay, and you didn't previously pay your restitution.