In your original question you didn't say that someone else claimed him. You said that he filed his own tax return. That's different. Did you ask him if he filed a tax return? As I said, you might get surprised. It wouldn't be the first time that a 14-year-old filed a tax return without discussing it with his parents. Or maybe his other parent or another relative filed a tax return for him, even though he doesn't live with them. Not that they claimed him, but filed a tax return for him, in his name.
If you can't find out who filed the return, file your tax return by mail, claiming him as a dependent. Don't forget to sign the printed return. Eventually both you and your dependent will get letters from the IRS asking for information to determine whether you can properly claim him as a dependent. The letter to your dependent will go to whatever address was on the tax return that he filed or that was filed for him.
One other, less likely, possibility is that someone with no connection to you made a mistake entering his own Social Security number, and it happened to match your dependent's number. If that's what happened, the IRS will straighten it out. It may take several rounds of correspondence with you.
Be sure to respond promptly to any IRS notices that you get.
