Welcome to Android Central! Try using an OK Google command: "OK Google, show me my last messages." Google should then offer to read you your last 5 messages and also give you prompts to dictate a response. This works on the phone, but I'm not sure if it'll work via the headset.
To make sure the Google app is properly set up for this, open the Google app, tap Menu>Settings>Google Assistant Settings>Phone>Other Hands-Free Settings, and make sure Bluetooth Devices is turned on there. Also, under Other Voice Settings, make sure Bluetooth Headset is turned on there as well.