The Secret Life of the Underwear Champ by Betty Miles, 1981.

Ten-year-old Larry lives in Connecticut, but one day, while he and his mother are visiting his dentist in New York City, he gets spotted by the Zigmunds.  The Zigmunds own a modeling agency, and they think that Larry will be perfect for a series of tv ads.  At first, Larry is kind of excited about the idea of being on tv and earning extra money (maybe enough to get a new ten-speed bike!) until he realizes what he’s going to be advertising: underwear.

The Zigmunds like Larry because he’s a clean-cut, athletic kid who likes to play baseball, and the advertisements are supposed to feature a family playing sports together . . . in their underwear.  Larry also happens to have red hair, just like the girl already picked to play his sister in the commercials, Suzanne.  Suzanne has been in advertisements many times before, and the idea of advertising underwear doesn’t bother her at all.  The underwear kind of looks like athletic clothes and isn’t really revealing, but it’s still underwear.  Larry goes from feeling proud of his new tv advertising career to hoping that no one at school ever finds out about it.  But, how can Larry even hope for that when his underwear-clad form is going to be displayed on everyone’s tv set?

Now, Larry is wondering what he’s gotten himself into.  He worries about his filming schedule conflicting with baseball practice and makes up excuses about needing to visit the dentist when he has to film a commercial.  Money or no money, Larry just wishes that his life would return to normal!

After the filming is over, Larry starts to feel better about what he’s accomplished.  He and Suzanne have become friends (and she may possibly be his first girlfriend), and learning about the world of advertising was kind of fun.  But when the commercial actually makes it to tv, and Larry tells his best friend Robert about it, the dreaded teasing starts.

In some ways, Larry’s fears about teasing don’t turn out to be as bad as he fears, although it seems like it at first.  Robert laughs at him when he sees the first commercial and realizes why Larry’s been sneaking around and making excuses, but Larry tells him off for being mean and challenges him to think how he feels about it all.  Robert feels a little bad about laughing but says that he can’t help it and that other kids at school will react the same way.  He’s partly right, but he does help to put a stop to it, and the other kids do calm down.  Larry even enjoys some minor fame because he’s the only one of the kids to have been on tv at all.

There’s a lot of humor in the story, but it’s also surprisingly thoughtful as Larry considers why people find the idea of seeing someone in their underwear so funny.  After all, everyone wears underwear of some kind, even the President of the United States.  It’s a normal part of everyone’s wardrobe and a common part of everyday life.  The other people in the ad don’t act self-conscious while they’re being filmed in their underwear because it’s just part of their job, another part of daily life.  The book doesn’t mention sex, although the “mother” of the family in the ads gets a few whistles when she’s in her underwear, and Larry acknowledges that he and Robert sometimes giggle over advertisements with girls in their underwear.  Larry’s main conclusion is that people laugh about underwear because that’s just not how people normally see each other, so it seems weird.  After everyone has seen the commercial with Larry many times, people get used to the idea and it doesn’t seem so weird, so they get over the “funny” part and stop laughing.  Eventually, the other kids at school stop thinking so much about the underwear and just think it’s kind of cool that Larry was in a commercial.

The book is currently available online through Internet Archive.