Can’t get verification code

My son (age 8 and part of my family sharing group) broke the screen on his iPhone 5. It’s not worth getting it fixed so hes trying to sign in to with his Apple ID to another device. It’s asking for a 6 digit verification code before he can sign in. But we cannot get that code because the screen on the phone is non-functional and the trusted number on the account is my old phone number. I didn’t realise I needed to change the trusted number on my kids devices as I thought they were connected to mine. How can I get him signed into iCloud on a new device without access to his broken device or the trusted number? Can I change the number directly with Apple as his Apple ID is linked to mine through family sharing??