Query about Nationality Identify for a child overseas

[barokut]

Hiya.

I'm typing this message on behalf of my British father who has a second wife in Bangladesh and currently in the process of applying for a British Passport for his son from that second wife. Below is his message:

"Hi there,

I have applied for a British Passport for my second wife's son. I submitted my British Citizenship Certificate, mine and my second wife's passport to the Passport office. And I also submitted my son's Birth Certificate, marriage certificate, family photos and all other relevant documents and evidences.

But Passport Office want to interview us to provide them identity and nationality of the son. They want to interview my wife in Bangladesh with British Consulate office and they want to interview me here in the UK.

May you please explain what does it mean by Identity and Nationality?

I'm looking forward to hearing from you soon."

The son never held any passport in his life thus we are unsure on what to do. Please help him in this matter. Many thanks