K1 visa valid for 90 days or 60 days?
 

I am confused about the I-94 stamp. My fiance has arrived on aug 20th on K1 visa and at POE he got a stamp saying "Valid until Oct 20". Just a day back we have realized that his k1 is valid only for 60 days(aug 20- oct 20). I am not sure if its the immigration officer's fault or its the normal process. Once the fiance enters the United states K1 visa is valid for 90 days but not in our case. My questions are.
1. we r getting married on sept 21st. what do we do in case we don't get our marriage certificate by oct 19?
2. Is there a way we can change the stamping on I-94 to reflect the 90 day validity of the visa?.