You may laugh, but I have really been a stickler about clothing and trying to make my son "fit in" at school. Underwear is kind of a big issue, especially if boys are changing for gym, and will see each other's underwear.
Briefs are comfortable, but tight whiteys are possibly the kind of underwear that will single a guy out and make him a possible target for bullies. You may be lucky and your son may go to a school where this is an issue, but my son is not lucky that way. Boxers are what the kids wear at his school.
Having said that, Hanes (and most other brands) make a cotton, soft, knit version of briefs that fits tight-ish but looks like boxers. I would try those. I've purchased them at Walmart and Target, and there are plenty to choose from.