How does he sign in with his old Apple ID?
iPad Pro, 11-inch, 4th Gen, Cell 5G
Sign out with the information for the new ID and then sign in with the information for the old ID. It sounds to me like you are trying to create a new ID, not just sign in and out.
Read this document to learn how to sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support
Click on this link to learn how to sign in with your Apple ID --> Sign in with your Apple ID - Apple Support
1. Open the Settings app.
2. Tap Sign in to your [device].
3. Enter your Apple ID and password.
4. If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in.
If another Apple ID is signed in you may need to enter the password for that ID to sign out of that ID first.
Sign out with the information for the new ID and then sign in with the information for the old ID. It sounds to me like you are trying to create a new ID, not just sign in and out.
Read this document to learn how to sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support
Click on this link to learn how to sign in with your Apple ID --> Sign in with your Apple ID - Apple Support
1. Open the Settings app.
2. Tap Sign in to your [device].
3. Enter your Apple ID and password.
4. If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in.
If another Apple ID is signed in you may need to enter the password for that ID to sign out of that ID first.
The person at the ATT store made my husband a new Apple ID. He would like to use the Apple ID associated with his iPhone but when he tries to sign the error message says that email address is in use.
