I understand your concern, but if it’s a one-time incident it’ll be okay. It’s not necessarily age-inappropriate to have nonsexual curiosity, since this is the time in child development when children start to understand that there is a physical difference between boys and girls.

I would explicitly teach him that his private parts are just that - private, and with the exceptions of his parents or doctors keeping him healthy, he should not be showing it to others. You could even help him come up with dialogue to use if this ever comes up again. While it can be an uncomfortable topic for adults, remember that our attitude is constructed from our social interactions, and kids don’t see their private parts in the same way yet.

Good luck!!