Scott, I'm sorry to read that you may have a bad product key in hand from your purchase at Fry's. There should, however, be no need to uninstall OneCare from the PC as it will continue to be in trial mode if you did not complete activation. The PC will have the latest available version of OneCare installed, no matter if you installed from CD or from http://onecare.live.com
You can send your son off with OneCare in trial mode and then contact support to get this resolved. If he returns within 90 days, you can activate for him with your LiveID and the product key. If he won't return, you can activate with the product key (assuming that you are advised to exchange the retail copy or are given a new key to use if they confirm that the one you have is invalid) at http://onecare.live.com/activate and then have him activate with that LiveID (and password) wherever he is.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve