Which citizenship route will my father need to take?

Hi all, I posted ~2 years ago and found out that I am eligible for citizenship by descent through my grandfather.

I was told to get my dad to apply for a passport FIRST, then I should apply direct to passport after he got his, as this would be quickest way to obtain citizenship for myself. However, this wasn’t possible because my dad had a job where he couldn’t have dual citizenship.

So, I’ve spent the last 2 years collecting documents I would need to apply on my own through BVA.

HOWEVER, my dad just retired from that job and can now hold dual citizenship, so he wants to apply for German citizenship himself ASAP! This is great news because I may not have to apply through BVA and wait years for my own citizenship 🤣

Here is his info:

His grandfather: born 1915 in Germany to German mother and father Emigrated/Naturalized: Never.

His father: born out of wedlock in 1939, Germany, to German mother and father, Emigrated to US: 1958, Married: 1960 to foreign wife (US citizen), Naturalized: Never (we have the non existence document from USCIS).

Self (so my father): born 1963 USA, Married: 1997 to US citizen.

Is it possible for him to apply direct to passport with success and what documents would he need? NOTE: His father has since passed, and we only have a scan of his passport and his Personalausweis. Can he apply without that?

However, I do have all other necessary documents (including Melderegister proving citizenship) that would have been necessary for me to apply independently from him through BVA.