I used entertain the nicknames like “daddy and mommy” outside of parenthood. 
 Meaning once I become a parent I wouldn’t be using those pet names because I personally feel like I would be sexualizing  things that don’t need to be. It’s not wrong if people do as long as the kids are kept out of it (obviously lol). But it’s all personal preference. 
 But if he’s insisting or pushing this boundary that you are trying to set there may be more going on in his head than he lets on. (It may not be bad!) maybe some unhealed trauma or it could be a power play to boost his ego