My fiancé and I need help. Sorry that this post is so long!

We have dated for almost four years. We are getting married on 6/23/18, our four month anniversary. We had to get certified birth certificate copies for when we get our marriage license. Obtaining mine was easy. I live an hour away from the city hall. We live in ny near Lake Erie. He was born in NJ and moved to NYC when he was four. We sent NJ the info and money they wanted for his birth certificate and waited. They sent it all back saying they couldn’t find him in their records. We tried doing it online, paid the fee and submitted all papers and notarized documents. A lady from NJ vital statistics called him and asked “what was your dad’s name?” My fiancé had to tell her his grandparents raised him (his bio mom’s Mom and her second husband) and that he never knew his dad. He had to call his grandparents and they looked up his custody documents where his dad signed his name. He found out and called vital statistics back. The lady said yes, that was his dad. But my fiancé has been using the wrong last name his whole life! His bio mom’s maiden name was Kender, my fiance’s grandma’s first husband’s name. She had my fiancé in NJ and his grandparents got custody when he was 7 months old, then she and his dad got married. She then changed his last name to their married last name which is Tanzer when he was ten months old. She never told anyone! My fiancé is happy to know this because he never liked his uncle (he is slightly an abusive drink but lives in Europe so he isn’t here much) who is the only person he knows with the name Kender. He wanted to take his grandparents’ last name, Bruce, because they raised him. They want him to keep Kender, but that’s the name he likes least. He would take mine but I’ve always wanted to change mine when I get married.

Any idea of what we should do? We have to fill out the application for our marriage license in a week or so. Help!