Do your own research about natural cures. Curcumin, grape seed extract, fenbendazole, maybe even ivermectin (where have we heard about that before??) Big Med and Big Pharma will tell you none of that stuff works.
First step, quit drinking, then get a positive attitude.

He has nothing to lose. I wish him (and you) well

Let him be happy. It's his life.

That being said, a relative has lung cancer and exhausted chemo and radiation. Then it spread throughout their body. They are now taking gene therapy, and it's working.

Cancer is completely beatable.

https://www.youtube.com/watch?v=P--XSuejl_o&t=405s&ab_channel=anonymouslegion

Look into Rick Simpson

Look into mushrooms and cancer, Chaga and Turkeytail mushrooms have been shown to have anti cancer effects, though I believe they are really just super healthy for your immune system which does kill cancer cells when healthy!

Get him off the alcohol!

Only God knows. If he gives his life to Jesus, it won't matter how long he lives.

It depends on a lot... but assuming mixed glioma which is kinda the worst you can have for brain cancer, you can expect a return in 5-10 years after remission.

usually just after they use all there saving fighting it

Everyone dies. Tomorrow is not guaranteed for anyone. Prolonging life for sake of life is pointless. Avoiding death is not the same as living.

Not to discredit your situation, but I've never seen anyone sick with cancer, that wasn't "told" they were sick with cancer.

My grandma was healthy as fuck until her doctor told her she had cancer. Literally just hearing the words made her sick with cancer. She went from working in the garden to dead in one month.

Your brother has the rest of his life left.

I'd guess a bad diet would actually slow cancer, since cancer is just you. If it's bad for you, it's bad for your cancer.

Cancer could already be there and just not causing problems, unless his blood work says otherwise. Even then, he could still have it and they're missing it.