Just be mindful tell a 7yr old he should know better will lead to lying and possible more hidden exploration. As adult we know a 7yr old should know better...but depending on the 7yr olds purpose he really may not have meant anything too harmful.

I've known other kids this age and older that have tried to play "doctor" with their friends. They just need to talked to, clarify what is and isn't acceptable and make sure he is getting the information he needs from his family (or you) so he isn't resorting to making other kids go naked so he can figure out what body parts boys and girls have and why they are typically so secretive.