Son (Petitioner) petitions his parents for Green Card. Both parents already in the U.S. legally. When parents are called for an Interview with USCIS,

Hello, Although some petitions filed by U.S. citizens on behalf of their parents are approved without an interview, it is always a good idea for the petitioner to be ready to attend the interview with their parent in case the Immigration officer wants to ask them a few questions. Good luck.   Disclaimer: this information is of a general nature and is not, nor is it intended to be legal advice. For legal advice about your specific situation please consult with your local immigration lawyer.