@Dinkster - Your best bet would be to have him in on the purchase so you can use his e-mail address to get signed up. If that's not an option, you can use your e-mail address. If you're using Google, you can do something like YourRegularUsername+sonphone [at] gmail.com (adding the +sonphone) to make it a separate e-mail address, but it will still get routed to your inbox. Once you get it all signed up and give it to him, you can work with Customer Care to get the e-mail address updated on your account.
Hope this helps!