Look at the bright side. It's going to make a ripper story for his 21st.
You've got to be proud the way he handled it, every now and then when you've just about had the little buggers they come up with something like this which lets you know that perhaps you're not doing such a bad job (as a parent) after all.
Buy him a stud finder as a show of appreciation and to assuage your feelings of guilt.

Deano