Short answer: no—iOS doesn’t let you lock Messages to “Keep Forever” or block deletions.
What actually works:
- Set a Screen Time passcode and lock Account Changes/Passcode Changes so he can’t sign out of iCloud or kill backups.
- iCloud route: use an iCloud-based monitor (e.g., mSpy). You’ll need the Apple ID + 2FA, iCloud Backup ON, and usually Messages in iCloud OFF so texts land in backups; once pulled, the dashboard keeps them even if he deletes later.
- Old-school: every week, do an encrypted Finder/iTunes backup to your computer and use iMazing to archive/read messages.
- Mirroring: Text Message Forwarding to a Mac/iPad you control can show texts in real time, but he can turn it off.
- Quick checks: your phone bill shows who/when (not content). Also consider Screen Time’s Communication Limits and Downtime.
Not glamorous, but backups + a Screen Time passcode are what stick in real life.