Maybe "Bubba" is offensive, and maybe it isn't. I tend to think it isn't very offensive if at all. I am a southerner, but have spent appreciable time in the frozen north country as well.

But I know for sure that "asshole" is offensive, and his mother should have corrected that error.

If you want something scrupulously neutral, perhaps "young man" would work. My father-in-law used "lad," but that is a little old fashioned.

I guess it's impossible to look at a chart and use their name....?

Not excusing his shitty response, but if someone called me Bubba you'd immediately get filed into the "IDIOT" folder in my brain.

If I spoke to an adult that way when I was that age I'd need bus fare to find my teeth.

I don't think that in almost 30 years of practice I've had anyone address me that way. If I did, I do have the luxury of booting them out (my clinic my rules), I suppose, but I really have no idea how long it'd take me to absorb what had happened. Certainly, I would not let it pass without immediately being addressed and corrected by the parent.