"I consider myself pretty fashionable, but my boyfriend wears tank tops and running shorts everywhere we go! How do I get him to wear better clothes?"
Have you asked yourself whether your boyfriend's fashion foibles are a relationship deal-breaker? If not, then your desire to discuss this may seem capricious. But if so, then sit him down for a heart-to-heart about the semiotics of clothes; that is, the clothes we wear send a message about how the world perceives us. He's dressing like a kid, which I assume he is not. Ask him to identify a fashion icon, someone whose style he could consider emulating (having some men's fashion magazines may help advance the plot). When he's able to express how he would like to present himself, give him some shopping assistance to help him attain that look. If, on the other hand, he's resigned to being a perpetual boyish slob, then let someone else befriend this Peter Pan.