Remove the keyboard, mouse and computer power cord and hide them.
He'll give you the password soon enough!
I should have thought of this when I spent three hours trying to audit a
friends Admin password with l0phtcrack after her evil son had done the
same. Fact of the matter was, I got fed up and her son was drunk so getting
the password was very easy.
On a serious note, to the original poster, it looks as if getting the
password back without doing a clean install is going to mean buying
software, be it some speacial password reset program. Lophtcrack might do
it, but I think it needs to be installed on the machine in order to crack
it, which I have a feeling requires administrator privilages.
Also, if you son is anything like the son of my friend who is constantly
doing this sort of thing, once you get this fixed you may need some help
keeping the system locked down. I have had my friends PC in my house for
three days after her son balls'd it all up again. Now that it's spent so
long here there's no way he's going to be screwing with it again. That's
why I'm subscribed to this group, to get ideas on how to keep it that way!!