During colder months, when my skin is dryer, I've masturbated my way to tiny lacerations around the edge of my foreskin.
Alternatively, you can wrap a string, shoelace, or similar item around the same section, mark where the ends meet, and measure the section of string with a ruler.
Really, he was just embarrassed by how small it was.
While it sometimes takes care of itself over time, when it doesn't, there are treatments for Peyronie's, so if you think you may have this -- though it's rare in younger men the average age it develops is around 55 , and only occurs in 1-3 out of every 100 men -- talk to your healthcare provider.
If you want to dig around for yourself, the kinds of studies our averages come from here can be found neatly organized in the notes for the Wiki on penis size.
Were you right in your estimate? Best wishes on your big adventure.