That I know of you can not register the drone to him.
All he needs to do is create his own DJI account, download and instal the Go4 app on his phone, boot the app and log in to it with his deatails and hey presto he can bet airborne.
Hello.!..You have to do exactly what the Yorkshire Pud tells you, there is nothing to do apart from what you yourself did when you first activated your Mavic Zoom. Luck.!