Is it possible that someone made a typo on their return and accidentally used your son's SSN?
We had this happen to us last year. When we want to file our taxes, we were told a tax return had already been filed under our social security numbers. It's unfortunately quite common. There are a number of steps you need to take, including filing a police report, a report with the FTC, the IRS, etc. Kind of a pain the butt, but pretty straight forward. The IRS has 180 days to investigate and release your return to you (if you have one).The steps are well laid out here.https://www.irs.gov/uac/Taxpayer-Guide-to-Identity-TheftSorry this happened to you!