You know those old sci-fi movies where everybody wore the same clothes? They were all written by males. Because most boys would rather never think about what to wear. Help a l'il brother out by restocking his activewear on the downlow. If you do it right, he'll never even notice.
Unless he finds himself in a Lord of the Flies situation, or the ancient Olympic "uniform" comes back in style, your Junior Mister has to wear something while he's out snipping snails.