I would take the letter to a CPA or attorney to review it. The issue is probably that it is not your son's fault or problem, the issue is with the fact that two taxpayers simultaneously claimed him which cannot happen. Do you have an ex spouse perhaps?
As long as when you claimed him as a dependent he properly qualified on your return then you should have no problem, but it is also possible you will receive a letter or audit. I would check your tax return again and also check with your preparer if you have one to make sure you filed properly and can properly claim him (as your son you likely can but there are a number of tests that determine what a dependent is). You may never know who tried to claim him as a dependent but they are probably in the wrong and could have even been engaging in fraud.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature.
Visit Steve Zelinger's website: http://www.stevenzelinger.com/
You won't be able to find out who else claimed the dependent but you can ensure that you remain entitled to claim the dependent. You may want to file a report with the IRS regarding the duplication to alert them that someone else is improperly claiming the exemption.
Evan A. Nielsen
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Evan A. Nielsen is licensed to practice law in California. The information provided here is for educational purposes only and is not intended as legal advice for a particular matter. This response does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult an attorney.
There is a test for a qualified dependent based on whether he receives more than 50% of support from you and is in your custody. Frequently divorced parents both claim a dependent, unless this is delineated in divorce paperwork. The IRS will eventually disallow one of the persons claiming the dependent. You should write a letter to the IRS documenting why you are entitled to claim the dependent.