Start a new user admin account, log into it, and erase the original account from there.
Not only can you not fully change the name like you're asking, I'd suggest this anyway. Who knows if he downloaded some weird driver that changed a setting somewhere?
Now that I think about it, I'd actually consider reformatting the whole drive. The biggest problem with the last OS update was for people who had logitech mouse drivers installed. It's going to be very hard for you to figure out similar problems in the future if you don't know what was done to the machine.
But if you don't want to re-format the drive, at least start a new account. That's a pretty good solution and takes almost no time.