overseas British Passport application over 21months and still waiting.

Hey guys can anyone please give me some advise.

my cousin brother applied for he's first British Passport application from Bangladesh.

In total the application has bein 21months now.
he's passport interview was done in November.

I letter of formal complaint was sent to hmpo complaint department, still waiting for a response.
what else can we do?
has anyone else waited 21months??

How has he gained British citizenship??

You will need to provide more history if you want any useful advice.

He's father was British citizen. age 18 now.
father is deceased.
A home office status letter was issued in 2017 stating he's a British citizen under section 2 1a act 1981.
he's interview went fine back in November 2018.
In total 21months and 1week now.

A complaint was made, hmpo waited the full 14days to reply and said they well get intouch within 10 working days, on the 10th day they replied to say they well need 10 more working days to give a update on the application.
Has anyone else had this experience?