I'm sorry that trying to activate is causing you stress. In order to activate, you'll need to LiveID that was used on your son's PC. It can be found on his PC by opening OneCare and clicking Help/About... - it will be the subscription ID.
If you can't get that from him, you can contact support by email through the instruction here -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
With the retail key from the package or your credit card, if the original subscription was purchased online, support can tell you the LiveID.
-steve