请教8老师：
I applied for employment-based immigration petition (EB-1A) and list my wife and son as benificiaries in I-140 (they were both in USA at time of I-140 submission). It is indicated in I-140 that my son will go through the visa processing instead of I-485. After the I-140 application was approved, my son went back to China before my wife and I submitted I-485 aplications (still pending with USCIS). My son has been staying in China since then. My questions are: (1) What should I do to initiate the immigrant visa process for my son? (2) Which agency should be contact first, USCIS or NVC?
谢谢！