Multiple iPhones and iPads with same Apple id
Sure I can help, but this will take time to cover all of the bases. I'll keep updating this answer as I update it. I'll even comment when I update it.

1st thing 1st you want to make sure that all of his devices have a backup. While this may seem tedious it's a crucial step in migrating and changing accounts and devices. We want to do this 1st so that nothing is accidentally lost, locked or disappeared. You can either do an iTunes backup over the computer, although I personally recommend an iCloud backup since its 1 less device that you have to open and use, and you can do it right on the iPhone/iPad as long as it has WiFi access and is charging/has a decent amount of battery life.

To backup to iCloud, 1st make sure that he has iCloud on in settings (Settings -> Click his name -> Click on iCloud -> Scroll down to iCloud Backup (if he's logged in he should see it) - Click on iCloud backup and then make sure its on (green) and then Backup now (unless he has a backup and its recent with no changes made on the device's information - aka: contacts, messages, photos, etc). The other way (for older devices) is to go to Settings -> iCloud and then iCloud Backup.

If thats done with all of his devices then the next step is to copy all photos on these devices to a PC. It's easier that way rather than to deal with iCloud. If his iCloud photo library is on (Settings -> Click his name -> Click on iCloud -> Click on Photos -> Make sure iCloud Photo Library is on aka green) then he may not have direct physical access to his photos since the cloud has already begun uploading them. If there are photos that he can't access from his device but they are on the iCloud, there is a way to also download them from a computer. He will need to know his Apple ID email (usually in settings -> click his name -> email thats under his name) and password.

If you aren't familiar with extracting photos from these iPhones/iPads leave a comment and then we can discuss those matters.

iCloud Backup - https://support.apple.com/en-us/HT203977



Once the iCloud backups and the photo extraction is done, then you are ready to check his accounts and see if he has a Gmail, Yahoo, AOL email (basically 3rd party client that hosts their own web-based email). If he does then usually that email has his contacts saved to it. He can always check by signing into the email address on the web and then checking contacts. If all of them are there great, then we can move on to the next steps. If not then we need to make sure contacts is on in iCloud so that they are all backed up. There is a way to download your contacts list from iCloud on the web. If you need that pointer, let me know. Once those contacts are confirmed/backed up then we can move forward.