How can I unlock an iwatch with no information

You can ask apple to remove the activation lock online on this page;

Online Activation Lock Removal Here.

Or this;

If you can't remember/gain access to the apple id that paired the watch to the phone.Take the watch and it's receipt with the serial number along with some photo id and any other proof of ownership to an Apple Store and request that they remove the Activation Lock for you.

You will need his Apple ID and password to release the watch from that Apple ID. Unfortunately, this is probably why he returned the watch to you without much of a fight -- he knew it would be nothing more than expensive jewelry to you.

There is one tiny sliver of hope. If you retained all of the original sales information, with your name on the invoice, Apple may agree to release it for you. Tiny, but more than the zero chance you have now.