I found this Info but some users had trouble with it, doesn't mean you will, so it might be worth a try. I think the easiest option would be for your son to set up another skype a/c from the computer he will be contacting you from and leaving the existing skype a/c on your computer for you to use.
Best of luck with it.
How to Run Two Skype Accounts on the Same Computer
--------------------------------------------------------------------------
Requirements: Windows XP, Two Skype User accounts
First, Create a new Windows user by going to the Control Panel and Clicking on "User Accounts". Choose "Create a new account" and follow the steps to set permissions. A Limited User is all that is necessary. I also suggest setting a password for the new user by clicking on the newly created User and choosing the Set Password option.
Next, create a shortcut to Skype or make a copy of the Shortcut that is already in the start menu. Right click this new shortcut and choose properties. On the bottom of the first page will be a button labeled "Advanced...". Click this button and select "Run with different credentials". Select OK, and then select OK again to close the Properties dialog.
You're all set. Whenever you use this new shortcut it will ask for a Windows username and password. Enter the username and password of the user you created earlier by choosing "The following user:" option. Click OK and you'll have a second instance of Skype. It will require that you log in with your Skype username and password the first time.