There is no REQUIREMENT to wear underwear under board short. When I see guys wearing boxer shorts under them its like, what up with that? Not only don't the not hold you in place, can you imagine what cotton boxers would feel like when they're wet and full of sand? uhh, no thanks. I usually wear Speedo briefs under my board short. Keeps everything in place and also keeps the sand out of my bu** ;)
your brother is right because keep everything in its place. You can go to beach without wear underwear you will get the answer why you should wear.
Your brother is right. because it also be comfortable and relax to keep everything in its place..