First passport and birth registration for Child born overseas

Hi,
I am White British so is both my parents. I am married to Bangladeshi citizen who holds UK residency permit currently. One of our son was born in Bangladesh. Should we register his birth in the UK before applying for passport and is it mandatory! How do we apply for his British passport and what documents needed. He is 9 month old. Please guide me if I am posting in the wrong place.
Thanks for your help

Registering a British child born abroad is not mandatory as you can apply for a British passport with the local birth certificate.

Explained here: https://www.gov.uk/register-a-birth

For passport application these links will give you details of what documents are required:
- If child is still abroad: https://www.gov.uk/overseas-passports
- if child is in UK: https://www.gov.uk/get-a-child-passport