If the previous owner doesn’t remember the old Apple ID and password how do you unpaid the old phone from the watch?

Tell him to check his web browser autofill entries, the Keychain, and other records, or use Apple's iForgot service, or contact their Account Security team, or the unit's original owner to click here and submit an unlock request.

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

Online Activation Lock Removable Here.

Or this;

If he 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.