And why should he have any shame in showing off his body?  All the gymnasts I've known have had great bodies, and I can't remember anyone of them who didn't take the opportunity to "show-off."  In addition, I can't remember any of these gymnasts wearing a dancebelt; they seemed to wear jockstraps in practice but often wore nothing under their leotards.

You have to explain to him that dancewear is functional, and it's important to the teacher, so that his muscle positioning can be seen.  Maybe he has other concerns about wearing tights in class, like how the ladies might view him.  You need to discuss his reasons with him.

Maybe he objects to the color scheme?  (Just kidding, although black and white aren't very imaginative!)

When the "pros" take class, many start with "warm-up-like" wear, including leg warmers, sweat pants, etc., but, by the time they're in center, most of that gear has been tossed away.  As for the photos in Dance Magazine, those have different selection criteria.  For example, dancers in sweats and all look more like typical athletes, and publishing those sort of photos is to enhance the "we're all one" attitude.  Another example, there's a famous male danseur who doesn't like pictures of himself that show his butt, and discourages their use.

The quick answer to the guy who points to these dancers would be: When you get to their level, you might be able to wear sweats, too, at the barre!