Some Calendar Items Don't Show on iPhone

Hi,

I have a user who's calendar is shared with his secretary.  She often adds invites/entries to his calendar.  He receives the email notification about a new invite and accepts them.  Most of his calendar entries show up in his iPhone but sometimes a meeting does not show in his iPhone though they still show up in his Outlook Calendar.  We're on Exchange 2013 CU8 and the user is on Outlook 2010, IOS 9.3.2.  Any assistance would be appreciated in helping get this calendar syncing to work 100% would be great.

Thanks,

John