Currently Being ModeratedJul 1, 2013 3:37 AM (in response to Joan1962)
Welcome to Apple Support Communities
That's the user name. It's a bit difficult to change it and you may damage your user, so I recommend you to create a new user for your son. Also, he won't have your files in his user.
To create a new user, open System Preferences > Users & Groups, press the + button at the bottom left part of the window and follow the steps. I suggest you to create an administrator user because he will be the only user.
After that, go to > Log Out, and log in the new user. Open System Preferences > Users & Groups, choose your old user and press the - button at the bottom left part of the window to remove it. With this, your son will have a new user that he will be able to set up as he wishes. You have to give the password of the new user to him