I'd do these 4 things:
1. Open iTunes and do a full backup. You may also want to log into icloud and download all of your contacts somewhere in case things somehow get messed up. (I keep mine on dropbox in case I ever need to access them)
2. Turn iMessage off ( settings > messages > iMessage off ). There are some reports of imessages showing up on the wrong phones for a while. It's probably rare, but worth doing.
3. Finally, wipe your phone entire. (settings > general > reset > Erase All Content and Settings). Make sure you pick content & settings and not just settings.
4. Go ahead and remove the SIM for peace of mind. He won't be able to use it, but can't hurt to take it out.