iMessage delivering to more than one device on same account
I have my son's iPod touch linked to my iTunes account.....but any iMessages I send or receive are being delivered to it...I obviously don't want them to be!!! I know I can turn off iMessage on one or both of the devices...but surely I shouldn't have to do that? He's only 9, and kinda likes the idea he can message me whenever he wants! Can anyone help? Thanks in advance.
iPhone 4, iOS 5.0.1