File, identity, new identity sets up a new identity.
The email address is arranged between you and your ISP. Most ISPs allow multiple email addresses, so it might just be a case of setting one up with them. As you probably know, you then set up the account in Tools, account, mail, add. You could do this without setting up a new identity. It depends on how private you want your son's emails to be - the new identity can be password protected.
Windows xp is good at handling multiple users so your son could log on to his own OE in xp if that is your OS.