Restrictions passcode

You erase the device using Find My iPhone or by putting it into recovery mode and restoring iOS using iTunes.

You will have to set it up as a new device. Do NOT restore a backup made after the restrictions were enabled or you'll end up right back where you started.

Photos can be extracted to a computer for storage. Contacts, calendars, notes, etc. can be synced to iCloud or another cloud service.

If it's not blocked by restrictions, you can turn on messages in iCloud and the text and iMessages will sync back down when he signs back into his iCloud account.

You can't restore from a backup. Restoring from a back up is all or nothing, including the restriction settings.