I think that they'll market anything (thongs for little girls, even) and it is up to the parents to fight the fight of good taste. While we don't win every battle with my SD, it also helps to express to her WHY we feel x or y is appropriate. There was one year when we went to a lake and she had a fairly wide bandeau top and didn't want to wear the strap. We said not on the boat. She slipped the strap off when she was tubing and when she bounced out she almost lost her top. She listed to us after that. She was 15 at the time.
If you want to set limits per your household, it wouldn't hurt to ask them to wear something more appropriate around your sons. Your pool, your rules.
There are fine lines when it comes to clothing, but if you allow your child to wear a skimpy suit, then you need to accept that someone is likely thinking inappropriate or unflattering things about your child. I allow my daughter to wear a tankini simply because 2 pieces is easier for diaper changes, but they are and will be tankinis with length until she is much older and a bikini is appropriate.
I remember being 13. And I remember my mother putting her foot down about what I wore.
I think better than buying suits on clearance, buy plain, huge tshirts and have anyone in a tiny suit wear one in your pool. Michael's sells them regularly for $5 or less in colors.