Stolen Device Protection cannot be turned off even after triggering passcode request; face id not an option

I bought my son a new iphone for Christmas and need to turn in his old iPhone to AT&T for a credit. My son lives in a different state. He and his new phone are now in his home state. I am with the old phone in a different state. We tried to turn off Stolen Device Protection before he left, and it started a one hour countdown, but it did not turn off before he boarded his plane. So now I have a phone with Stolen Device Protection and Find My turned on. I need to get them turned off. Following other online advice, he went into icloud and removed the device. The status in his icloud for the old device now says ready for repair or trade in. However, the device itself still has SDP and Find My turned on. I have tried repeatedly triggered the passcode to turn off SDP, but it still wants his face id. I am concerned that if I erase the iphone I still won't be able to turn off SDP, Find My and remove his passcode. Any suggestions on how to get SDP off so I can turn off Find My? Thank you.