I'm sorry about this diagnosis. What information has his doctor given him?
Glioblastomas are a very aggressive form of cancer, and a cure isn't often possible, but treatment can help slow the progress of it.
Regardless of the size of the tumor, glioblastoma is a very serious, aggressive cancer. Is this your husband or father? Has anyone talked to you about the prognosis of it?
I'm very sorry to be the one to tell you this, but the survival statistics aren't good. At the end of 5 years, only 5.5% of those diagnosed survive. Remember that these are just statistics, though, and treatments are improving all the time. My father was told that he had a 15% chance of being alive in 5 years because he had a very aggressive form of prostate cancer, and he died 12 years later.
I'd encourage you to talk to his oncologist, who can help you understand what's happening, and what particular things you should look for, given the location of the tumor, the stage of the tumor, his treatments, his overall health, etc.
Go to Google and search glioblastoma. Make sure you understand what it is, and how serious it is. Talk to the oncologist (the cancer doctor). I'm not trying to scare you, but to prepare you. If you only have a short time left with someone you care about, knowing that can help you make the most of that time. It can also help you understand that you need knowledge. There are clinical trials he might qualify for, new treatments.
https://www.scientificamerican.com/article/why-is-glioblastoma-the-cancer-that-killed-john-mccain-so-deadly/ - this may take some time to get through as it is written by a doctor, but it may be really helpful.
Definitely be cautiously hopeful. Let us know how he's doing, and how you're doing. I wish you both the best.