Let's go over specifics. You are talking about iCloud mail yes? If not let me know.
If you open the mail app. At the bottom it should say updated or checking mail something like that.
In your inbox you want to pull down and release. You should see a buffering circle and then look for errors.
Start here: https://support.apple.com/mail
If you see account error you need to click it. It will tell you the issue.
Have you attempted to add your outlook inbox directly to your mail app? Settings>mail>accounts>add new account
https://support.apple.com/mail-settings-lookup
The above link can find the settings for most domains that direct mail to an exchange/outlook server
You can try using https://support.apple.com/mail-settings-lookup But chances are if you’re setting up a school or work email it may not be able to find the settings for you. You’ll need to look up your school’s and your work’s support sites for the server information, or call their tech support departments.
It is unclear from your post who your dad's email provider is. Is it an account he has through his local internet provider, or is with one of the services that allows for automatic configuration, like Google, Yahoo, Outlook? Who did he already speak to, someone at Apple or technical support for his email provider? If it was Apple, the best they can realistically do is refer to this page or this one and then once the settings are in-hand, assist with entering them. Chances are the email provider is only going to speak with your dad, but there's no reason you couldn't be on conference call with him and then dial the email provider's support number and merge it into a three-way call.
Apple hosts a website titled Mail Settings Lookup. Try the settings it yields. The Outlook app for iOS requires iOS 11.0 or later. If you want to see if an older version is available for installation you will have to “purchase” the app via iTunes on a Mac or PC then try to install it onto the phone by means of your purchase history.