My Boss's iPhone 4s calendar quit syncing with Outlook. Please help!

I have tried everything I could find on forums with no luck. iPhone OS is up to date, latest version of iTunes and even took Outlook from 2003 to 2010. Uninstalled iTunes, erased all content and settings from iPhone and his iPhone calender events will still not sync with Outlook events. It is a POP3 email account and cannot go to an exchange. He used to sync daily using iTunes to update calendars and I believe the contacts and email account sync just fine that way. Any suggestions would be greatly appreciated!