Hello Muttmutt,
Thank you for using Apple Support Communities.
I understand that all the mail you receive from your Dad is sent to the junk folder. You have tried a number of things already, but have not been able to resolve the issue. I would recommend taking a look at the following support article first to see if there is anything you have missed:
If junk mail filters aren’t working in Mail on Mac - Apple Support
You can also try this from the following article:
"Reset
Restore the default criteria for identifying junk mail.
Use this option only if Mail no longer handles junk mail as you expect. Resetting the filter deletes everything that Mail has learned from you about junk mail."
Change Junk Mail preferences in Mail on Mac - Apple Support
Best regards.