If you erase all content and settings it will wipe the phone and there will be nothing on it. If you want to change Apple IDs:
- For purchases, go to Settings>Store>Apple ID, tap the ID shown, sign out, sign back in with your ID
- For iMessage, go to Settings>Messages>Receive At, tap the ID at the top, sign out, sign back in with your ID
- For FaceTime, go to Settings>FaceTime, tap the ID, sign out, sign back in with your ID
- For iCloud, go to Settings>iCloud, scroll to the bottom and tap Delete Account. Then set up a new iCloud account on the phone with your Apple ID.
- For mail accounts, go to Settings>Mail,Contacts,Calendars, tap each account, scroll to the bottom and tap Delete Account. Then set up your own.
That should about do it. Note: for app updates, if there are apps on the phone that were purchased with his ID you will need to delete these from the phone and from your iTunes library (if there) in order to not be prompted for the password for his ID. (Apps are permanently tied to the Apple ID used to purchase them.)