extending student visa

My son has an X-2 student type and is studying at the Beijing Normal University this summer. He has an opportunity to study at Fudan in Shanghai from September to December. Although the X-2 type is supposed to be good for less than 180 days, his x2 says valid for 90 days because he did not think he would need it for longer than that. It expires in mid September. Would he be able to extend or renew his current one? Will he need to change it to an X-1 type? What would be the best way to go about it. he is a U.S. citizen.