IPhone :: Converting A Used 3GS To An IPod - IMessaging Not Working?

I just upgraded to a 4S iphone and am giving my 3GS to my son to use as an iPod. I've created an email for him so he can iMessage but it won't receive/send through his email. I've changed every setting I can find. I think its trying to use the phone number even though its no longer has phone service which is why its giving the "Message Send Failure"

I already had iMessage turned on from when I was using the phone as a phone... I created the email and verified it through Apple as an email I want associated with my Apple ID. I went into Settings>Messages>Receive At and added his email address but it won't let me delete my phone number (which is now tied to another iPhone device). I also turned off Send as SMS.I also went to Settings>Messages>Receive At> Caller ID and changed the check mark to be next to his email instead of my number.

I went in the Settings>General>Network and turned off 3G and Cellular Data settings (which I don't think were affecting this, but should be off anyway if he's using it as an iPod.)

I tried going to Settings>Phone and deleting my number out but it won't let the blank remain empty. I tried changing the number to 0 (zero) and then when I opened messages again it immediate popped up a message saying "MMS Messaging Requires Your Phone Number Press Settings to go to your Phone Settings and enter your phone number"

The only thing I haven't done is add this email to the Mail settings, because I really don't want him receiving or sending emails, I only want him to be able to text his grandma, cousins, his dad and me. But I'm wondering if the fact that I haven't actually entered his email account password anywhere (like in a pop server?) is why it isn't working.

Info:
iPhone 3GS, iOS 5.1.1