The second account must have its own unique User ID. It can be an email address (different from the other User ID), but it doesn't have to be an email address.. In the account settings profile, however, as VolvoGirl mentioned above, you can use the same email address as yours for notification purposes, if you desire to get the notifications about the son's account, or you can use the son's email address in the profile if you want him to get his own notifications.
