During the initial setup process when the 4213 was new, you should have been asked/prompted to create an emergency copy of the installation disc on a memory card.
Unless you did that, you can't revert to the original OS, unless you also have a legitimate disk copy of Windows XP, and a bootable CD drive.
You need to ask your son how he obtained and installed the Windows 7 operating system. Hopefully he downloaded it and ran it from a USB memory stick. If so, you may be able to go back and install it again, but use the boot tools to create a fresh installation by reformatting the hard drive. It may even be possible to do a 'repair' byt using this method.
If he used a file on the hard drive to update to Windows 7, I suspect you are scuppered, and will need to obtain a legitimate OS on a USB drive, or use a botable CD Drive to reinstall.
We have got a sister site dedicated to Netbook stuff (we originally handled all Netbook queries here but got a bit overwhelmed! It may be worth posting this on
4NetBook