The issue that you have is actually going to be a problem for you because there is no way to override his password.
Ask the previous owner to enter their Apple ID and password ***** the Activate iPhone screen to remove the device from their account. He can reset his password ***** their site.
If the previous owner isn't present, contact them and ask them to follow these steps to remove the device from their account:
After the previous owner removes the device from their account, turn off the device and then turn it back on to begin the setup process.
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