Wow, that’s not the response I was expecting 
Anyway …
This has nothing to do with re-installing Cubase 6? 
Quit saying “Cubase 6” if you mean Cubase “LE AI 6”. Cubase Elements/AI/LE is capable of using the Soft-eLicenser instead of the USB-eLicenser. I’m assuming this is what you are dealing with?
You need to authorize “LE AI 6” (if you are using the Soft-eLicenser) before you enter your Elements 7 activation code.
The Elements 7 code needs to “see” the “LE AI 6” registration in order to be validated.
Meaning … Cubase “LE AI 6” needs to be authorized (or “re”-authorized) before the Elements 7 code will be accepted.
If you can’t accomplish that due to confusion over MySteinberg accounts and email addresses, it sounds like a case for official support (through your MySteinberg account - or your son’s, hard telling).