How to prove Brazilian DNA as UK born

I was born in the UK and still live in the UK, and one of my parents is Brazilian. He left before I was born and I only got in contact with him after I turned 18.
His current wife and children don't know I exist because he claims he didn't know I existed either and he isn't ready to tell them. I've decided it's not my place to force that further as that's his family.

I want to obtain a dual citizenship passport just so I have it in case it's ever needed in the future, and I'm legally entitled to it as a child of a Brazilian-born, and currently residing citizen. I'm not sure if I can involve him though as he might say no in fear of his family finding out.

I assume there's a way for children of other nationalities to prove their heritage without the parent, in cases where they pass away before they're born so aren't on birth certificates or able to vouch. Does anyone know how I might be able to do this?

Brazilian citizenship is not inherited automatically if the child is born abroad. They need to have been registered with the Brazilian diplomatic authorities in the country they were born outside Brazil.

You could try to register your birth with the Brazilian authorities now. If that fails, the website continues;

Those unable to meet all consular birth registration requirements, having had their application denied, must make the following arrangements:

a) Get the document apostillised by the FCO, translated in Brazil and then transcribes the foreign birth certificate directly at a First Civil Registry Office ("Cartório de Primeiro Ofício de Registro Civil"); and

b) Officially opt for the Brazilian nationality by means of a non-contentious specific legal action, to be filed before the Federal Court of Justice.