Authenticate

You need another trusted device or telephone number.

See Two-factor authentication for Apple ID - Apple Support:

A trusted device is an iPhone, iPad, or iPod touch with iOS 9 and later, or a Mac with OS X El Capitan and later that you've already signed in to using two-factor authentication.

4 Ways to Change the Trusted Number for Your Apple ID on iPhone or iPad

Get a verification code and sign in with two-factor authentication - Apple Support

MaryCogs Said:

"Authenticate: can’t authenticate bc sons cell is destroyed. How can I authenticate his iPad without it."

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Is Your Phon Associated?

If your phone is associated to his account, see if he can add your phone as a trusted number. Go Here: Apple ID account page. If it is not associated, calling Apple Support is you only way around this

Contact Apple Support:

Elsewise, contact Apple Support, and have them run him through adding his computer as a trusted device.

A. Phone Support Info: Contact Us - Choose Locations