Is my child a U.S. Citizen?

I got my citizenship in November 2011 and before that my son (14 years old at the time) traveled to Liberia in October 2011 to see his sick father as a permanent resident or a green card holder: few weeks later I became a U.S. Citizen. By law since he was a green card holder at the time I acquired my citizenship, he was supposed automatically be an U.S. Citizen; therefore, I told him to stay (in Liberia and take care of his father and also complete a high school there since he has now acquired his citizenship through the process of derivation and no longer had to worry about loosing his status. I am now ready for him to come back to the States to attend college so I told him to apply for a U.S. passport , according to him he booked an appointment with the counselor at the embassy in Monrovia, and she said the not only is he not a U.S. Citizen but also he has lost is permanent residence status. Is what she is saying true and how is that? Note that he never came back to the States...