I am not able to give you the answer, but you can get it by contacting a local branch of Immigration, or maybe try talking to the people at the marriage license department at the court house regarding how you find out and where you go to find out. Depending on his immigration status, you may be able to get around the social security number issue. Or maybe there are steps he can take toward citizenship that will allow him to legally marry you. I’ve always heard stories of US citizens who married an immigrant in order fore that person to stay in this country. So there has to be a way. Just be prepared for close scrutiny and some hassles. Otherwise, let him apply for citizenship and wait to get married until he is a citizen.