My son is now 22, was a full time student in 2016, but graduated, got a job, and filed a tax return claiming an exemption. Can I still claim him?

He was a full time student and graduated in May 2016. He lived with me for more than half the year, and did not pay for any of his expenses. It says he now has to amend his return, or that I need to delete him from my return and cannot claim him as a dependant. The message I received about the rejection also stated I could not e-file, but could file by mail. Does my son still need to amend his return if I do this? What if he already filed, and received his return? Thank you.