Erase iphone

The device has to be able to receive the message from Apple to erase the content. To do that it has to be able to connect to the Internet. In this case I imagine it is unable to do so and since the thief has your passcode then most of your data can be read by the user of the device. While your account hasn't been completely compromised you may wish to read:

Click on this link "If you think your Apple ID has been compromised" --> https://support.apple.com/HT204145

'Checklist: If you want to see if anyone else has access to your device or accounts' click here --> https://support.apple.com/HT212021

Security and your Apple ID - https://support.apple.com/HT201303

Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169 "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues." Better for directly reporting an issue.

What makes you think he knew your passcode?

He might have simply performed a hard reset or pulled-out the SIM.

As long as he doesn’t know your passcode, the data on your phone is encrypted and can’t be read. (at least not by common criminals)

HOWEVER … be VERY WARY of any texts and e-mails purporting to be from Apple.

It’s a too-common way for the crooks to gain access to your device via “social engineering” … or worse … compromise your AppleID.