Yes, you will keep your accounts, and all of their saves, I'd suggest upgrading on the Xbox to make sure you don't accidently create a new account (very easy to do on xbox.com).
Just sign onto your profile, get into Account Management, and Go to Family Pack (4th or 5th window).
Then it will bring you into 'Family Center', Just go to the right to Add Family Member, and select his tag
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