How can I fix iMessage not syncing after resetting a family member's iPhone?

Hi! I reset my nephew's phone (he is in my family group) to factory settings in Find My app using Erase Device option. Then he re-logged into his account on the same phone. Later we found a problem: old iMessages are not showing up and iMessages sync status shows "Not synced" even after force pressing "Sync". Please help me to resolve this issue. Thanks in advance.

[Re-Titled by Moderator]