Son's New IPod instant messaging my account. What setting do I change?

Greetings,

I recently bought my son a new iPod. He is 8, so I have restricted as much as I can on the iPod. He can't access certain controls or the iTunes store or certain apps. It is as locked down as I can make it.

One odd thing that happened recently is that my wife and I were text messaging on our iPhones and my son replied to one of the messages. He was getting the messages my wife was sending to me.

The only thing that I can think of is that since I set it up under my Apple account (my son does not have an Apple account) so that I can sync the unit on my computer, that it "thinks" it is in my possession.

Can someone help me determine what setting I need to change in order for my son to not eavesdrop on my text messages to reply to any messages I receive?

He has his own email account, Just not his own Apple iTunes account.

Thank you,

TPK