DS is 6 he still wears lindex brushed cotton inside leggings (not to school at they're not in the uniform policy but for non uniform days, outdoor learning, playdates etc) and so do several of his male school friends, and the ones who don't don't bat an eye that others do. DS is very tall for his age and very slim, joggers fall off if I get them long enough and are pretty scruffy. I don't see why boys should have to wear jeans or trousers with a button fly.
Having said that I wouldn't put him in those shorts, I wouldn't put a girl in them either.