Welcome to the forum walle.
You need to be more specific about which version of Windows is on your son's computer. For example: Is it
Home, Professional or
Ultimate? Is it 32- or 64-bit?
Also, is the backup CD from your computer the exact same version of Windows as your son's?
You are in luck since there are free ISO downloads for Windows 7. Please see
this article on using Windows to repair your son's computer.
Make sure you choose the correct Windows version! Once you've downloaded it, burn it to a DVD (it's too big for a CD) and follow the instructions in the article.