Herstory: I'm very sorry to hear that someone you care about so deeply is ill like this. And I'm certainly sorry to hear about any kind of tragedy or other hardship associated with this new relationship that has been so otherwise positive for and so important to you.
There's a lot to potentially talk about here, but I want to start by voicing some concerns and with a couple basic questions.
It doesn't sound like this person actually has a clear diagnosis, prognosis or treatment plan, or if they do, it doesn't sound like they're sharing them with you. Someone diagnosed with any form of cancer of the brain is typically going to have a lot more information than it sounds like this person has (or is sharing with you), as well as some kinds of treatment, both for the cancer and its symptoms. It sounds like you're saying one thing that has you both so scared is not knowing what is going on; is not knowing anything, really, about what is happening here. But if this person has a medical diagnosis of cancer, they should have a LOT more information than it sounds like.
Can you fill me in more on the whole picture here? Is this a new diagnosis? Are they currently getting sound treatment and care? Even though this is a brand new relationship, it sounds like you two have gotten very close very fast, so can you go along with them to their next oncology appointment so, at the very least, both of you can be less scared and uncertain -- and also be sure you're even doing what you should be during times he's reporting things like massive headaches and loss of vision -- by getting more solid information?
Never doubt that a small group of thoughtful, committed citizens can change the world. Indeed, it is the only thing that ever has. - Margaret Mead