I grew up in Mississippi and while I never took a poll, I suspect most guys in my high school wore briefs when they were younger but moved to boxers later on. Mom bought my clothes and underwear so she just bought tighty whities by Hanes or Fruit of the Loom at Walmart.
When I started college, also in Mississippi, I was literally the only guy on my who was wearing briefs when I arrived. During week one I drove to the Walmart in that town and bought some Hanes or FTL boxers, can't recall. I packed my briefs up and then just tossed them later.
Years back a country guy from rural TN I played around with was a hanes white briefs wearer. Hadn't seen them on a dude in years and I just about ripped them off him lol.