Since the yeshiva bochrim are wearing white shirts and dark pants i think that the things that could be different (glasses, ties, shoes…just to name a few) should be ..well, not necassarily “in style” but nice. if a girl would wear certain clothes, it would show her personality. a boy can’t show his personality through the white shirts and dark pants so he should get nice glasses, or ties to show his personality! i think that that is very important.
also, just because something is instyle doesn’t mean that it had to be more expensive, or he had to be haning out in a mall to get it. he could have gotten it from a frum store and even if it wasn’t in style he might have to go the mall to get it so there is no difference!