V.W.
My son is 14 and likes the cotton boxers. He WAS wearing boxer briefs, but decided he wanted to go with plain boxers.
I don't think I would read anything into it. Maybe he is proud of his .... parts... and thinks it would feel sexy to have that kind of underwear. I mean... at 15 aren't their hormones supposed to be RAGING?
He may wear them a few times and decide he doesn't like them. The end.
Don't worry about this.
PS. What I normally do about a "change" in their styles like this, is to buy ONE package so they can "see if they like them" first. If so, I don't mind buying more. But sometimes a new style isn't all they thought it would be cracked up to be. Ya know?