Pairing a watch

You can't do anything unless you know his ID.

You can look up his ID here: https://iforgot.apple.com/appleid

You can reset his ID password once you know his ID here: https://iforgot.apple.com/password/verify/appleid#

There's nothing can be done.

Activation Lock is a theft deterrent feature with no bypass or workaround.

Without the owner's authorisation (Apple ID & password), no thief or anyone including you can re-use the device.

You can recycle the device responsibly.

See this;

How to request access to a deceased family member's Apple accounts