Actually, the sentence does not make sense. How could one possibly stand in a shirt? You can say "er trägt ein weißes Hemd." or "Er ist im weißen Hemd".
It's just another way of saying things. Trick of learning a new language - Not all things are direct translations. Just like we have different expressions and phrases in different regions.
Here's a perfectly good English phrase that makes no literal sense: "It stands to reason that........". English Stand and German Stehen are not the same word in every sense. Their range of meanings overlap considerably, but they also have some wonderfully surprising areas in which they do not, especially where idioms are afoot.
What about "he is standing with the white shirt", meaning "he is the one standing with the white shirt on" ?