You're inquiring about two separate things, reading email and setting up Apple ID. One has very little to do with the other.
The message you received means that there is already in existence an Apple ID with your email address as the username. For that account, you'll need to remeber or reset the password.
When you tried to reset the password, the verification got sent to the "rescue email" and that's why your son recieved it. Answer that email, then reset the password. The message about an another app benig downloaded is simply letting you know that your Apple ID has been used.