How do you...?

How do you tell the new guy that his bottom two buttons on his shirt have popped open? I mean I have been trying to figure out the nicest way to do it. But I can't. I want it to go away because it isn't like his stomach is nice to look at exactly, just the opposite. And I can't wait to see him in the summer. He's sweating a lot now, what will he look like in August? Ew...