He won't be able to access the actual data that was erased from the iPhone but if he has your Apple ID and password there are a number of other things he can do, such as accessing any data stored in iCloud including a backup of that device which might have a copy of the same data that was erased. You should change your Apple ID password. See:
Change your Apple ID password - Apple Support
and
If you think your Apple ID has been compromised - Apple Support