how do i restore cell service to my watch

I transferred my son's apple watch SE from being connected to my wife's phone to his own phone. During the process, I had to chose between "erase data" or "erase data and keep plan." The screen was small and I tapped the wrong one by mistake and now his cell service is not active. When I go to cellular on his watch app it give me the option to set up a new plan but not to reactivate the old one. The plan is not cancelled with Verizon and the number is still on my verizon account, but I cannot figure out how to restore service to the watch. Help, please.