In this case if you don't have another device with his account already logged-in, a trusted phone number for his Apple ID like your number or another family member’s, Apple can send the code via text or call: When prompted for the code on the new iPhone, choose "Didn’t get a code?" and select "Send to trusted number"
If this doesn't work, You can try to recover his apple account: when asked for the code, tap “Didn’t get a code?”
Choose “Can’t use any of these options?” and this just follow the instructions to start Apple’s Account Recovery process.
Apple may ask security questions or require waiting several days to verify identity.