Yes, you may still be able to claim him as your dependent but if he has already filed a Tax Return and claimed his own exemption, the IRS will reject any subsequent returns that contain his Social Security Number.

To resolve this you can have your son file an amendment (form 1040X) to remove his personal exemption. This will more than likely result in him having to pay back some of the refund he received (if he indeed received one). Please keep in mind that the IRS asks for 16 weeks to process amendments. Additionally, the IRS only accepts form 1040X by mail.

Considering how late in the Tax Season it is, you will have to file an extension or file your return by mail as soon as your son sends in his amendment. If you file your return by mail you will include him as your dependent. This will not be a short process as the IRS must acknowledge his amendment and subsequently grant you his exemption.

Below are the FAQs with instructions on amending a return, and the second FAQ which contains instructions on Printing your Return to file by mail:

 https://ttlc.intuit.com/replies/5114064

https://ttlc.intuit.com/replies/3401713