Short answer: there’s no truly invisible, real‑time iMessage reader on iPhone. Apple locks it down hard.
What actually works:
- Same Apple ID on a Mac/iPad with Messages in iCloud on: you’ll see all iMessages, but he’ll get sign‑in alerts and can see the device in Settings.
- iCloud backup method (mSpy/others): needs his Apple ID + 2FA, Messages in iCloud OFF, and no Advanced Data Protection; you get delayed snapshots, not live, and it breaks easily.
- Local computer backup: trust his iPhone to your Mac/PC, enable encrypted (or Wi‑Fi) backups, then a tool parses messages; needs physical access and occasional syncs, semi‑stealth.
Extras:
- Carriers/phone bills won’t show iMessage content (or even logs reliably).
- Use Apple’s Communication Safety, Contacts Only, and weekly spot‑checks + a clear agreement. If you want, I can walk you through any of the above step‑by‑steps.