See, I think bristling at being called "ma'am" or "sir" is a little weird. They are just polite pronouns for a woman or man. However, I have had a couple men tell me not to call them sir and laugh when I'm at work. It do it almost mechanically, and it's not that I'm assuming they are older! However, when I know their names, I always use that instead.
If someone called me 'ma'am' I would have no issue with it. It's like calling me Mrs. instead of Miss because I'm not married. An example of how pronouns are just used as polite terms is my mom. She has never liked the idea that she is "stuck" with a man, despite being married for 25 years or so to my dad. She tells people to refer to her as Miss rather than Mrs., despite being a married woman. Like that, how would anyone know if they are in wrong for calling someone "ma'am" if they aren't told?
If your friend's son calls you ma'am, just tell him to refer to you as Miss Maryellen, like you said, or however else you want to be refered as. He doesn't mean to do any harm with his politeness, nor do his parents.
