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That is because it is syncing with your iCloud account. On the iPhone 4S, go to Settings > iCloud and tap Sign Out. Tap Delete when asked. This will sign out your iCloud account from it. Then go to Settings > Mail and remove your email accounts from there.
Then go to Settings > iTunes & App Store. Tap your Apple ID and tap Sign Out.
Then go to Settings > Messages > Send and Receive and delete your IDs from there. That's it.
If you would like to delete all the contents from that phone and setup it like new phone, then all you have to do is, go to Settings> General > Reset and tap Erase All Contents and Settings. This will delete everything from it including your accounts and start fresh, so he can register it with his account.
If you have any questions or would like any additional information, please don't hesitate to ask.
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