Co-Worker Calls Me By My Nickname – Leo and Rebecca Buzz Question

Leo and Rebecca Buzz Question - Hey guys, HOW DO I GET A CO_WORKER TO STOP CALLING ME BY MY NICKNAME?.....My family and good friends call me this NICKNAME and all of a sudden he is?? My bestie does work with us and she calls me this but I really don't want him to. I've told him a couple of times 'Hey my real name is...But he doesn't stop. Any advice?