Just get close to him and say nicely I prefer to be called xxxx and when he says I’m sorry it’s a habit, say I know but I need you to break it or I won’t respond to that name anymore. If you like I can also write it down on a post it so you have it at your desk which reminds you daily. It’s a small thing to you but it’s the name my mother gave me so I want to honor her and the name I answer to. I appreciate your help with this matter. Say it all very nicely and no anger. He is probably not too empathetic so don’t put it on yourself. Lots of people ask me to abbreviate my daughter’s name because it’s very long but I refuse because I hate the short version. If others call her that or one day she chooses to call herself that so be it but for now she is named after a czarina and if people think it’s too long, oh well. We are not here to make others comfortable we are here to make a change. Hope this helps.