Hey jscoughlin,
From what we understand, the iCloud data downloaded to your iPhone was your son's data.
In order to resolve the issue, you will need to select the correct device when restoring the backup. In order to delete the current data and install your backup, you will need to to erase all content and settings. After doing so, you will have the option to set up the device again, and select the correct backup.
The steps below helps you reset your device to factory reset settings. We wouldn't recommend creating a backup of the existing information since its your son's data. It may replace the current backup that you have for your device.
1. Tap Settings > General > Transfer or Reset [Device]
Tap Erase All Content and Settings.
2. If asked, enter your passcode or Apple ID password
Forgot your passcode or Apple ID password?
3. Confirm that you want to erase your device
If you have an eSIM, you can choose to erase your eSIM or keep it. If you erase your eSIM, you need to contact your carrier to reactivate your cellular plan.
4. Wait for your device to erase
It might take a few minutes to completely erase your data.
Afterwards, set up the iPhone again by following the steps here: Restore your iPhone, iPad, or iPod touch from a backup.
Hope this helps. Take care.