Hi Rebecca!!
I hope you're having a super fine day.
Boys get erections INSIDE the womb!! Imagine that!! That's just something the penis does. It's what it was - in part - designed to do. The fact that the little boy's penis was erect when you changed his nappy proves one thing - and one thing only. IT'S WORKING!!
The next time that happens - do yourself AND that little boy a favour.
Ignore it!!
GREAT BIG HUG
Craig!!