Functionality of Apple Watch 5 Cellular w/o any other Apple product

Unfortunately, no. The Watch would be married to your phone, since it takes that to create the account and activate. It would be set with your telephone number, and calls to you would be going to his watch. The Watch requires its own phone for its number and operations.

He will need an iPhone to even activate the Watch, and to copy the SIM information from that iPhone to the Watch. The Watch can call 9-1-1 if the iPhone is not nearby, but you cannot activate a Watch on its own.

You couldn't sever the connection from the phone because the watch needs the phone it uses your sim card on the phone to make the calls to emergency services.