How to bring it up
Just like bringing up your insecurities, start with a story. What is it about your parent’s background/upbringing/culture that makes them so uncomfortable with this particular trait in your partner? If you need to explain their culture to him, or even struggles they had in their life that make them believe certain things, do that. You don’t want to make it seem like a judgment when you finally tell your partner what bothers your parents about him. You want to make it seem like a difference of beliefs, and you want him to see where your parents are coming from—whether or not he agrees—so he knows it isn’t personal. Finally, whatever it is about your partner that your parents don’t like, make sure he knows you adore that about him. In the end, that’s all he needs to hear.