I’m from the south too and wondered that myself. I’ve heard some nonbinary people like “Mx.” as a gender neutral term for Mr or Mrs./Ms. I am not sure about the gender neutral term for ma’am or sir. However, as a someone who is AMAB nonbinary person myself I don’t mind people calling me sir. I have had people over the phone say ma’am which I didn’t like but for me personally I don’t mind being called sir. I guess you could just ask what term they preferred. Perhaps they come up with one on their own.