I'm so sorry. My sister has a GBM, and I understand what it means, at least to me, to have a sibling affected by this horrendous cancer. Have the annual MRIs been due to a benign tumor from the past? My sister had a grade II astrocytoma in 2004, and the MRI that discovered her GBM was a routine follow-up MRI, 13 years later.
If his neuro-onc is "very uninvolved," can your brother seek another specialist? I'm not sure where he lives, but there are several cancer centers across the US. My sister is seen at Duke University Medical Center's Preston Robert Tisch Brain Tumor Center in Durham, NC. She receives all her treatments at her local cancer center in Columbia, SC, but it is guided by Duke.
Every month one survives, is a month closer to potential new treatments. Potential cures. I always keep that in mind. My sister's treatment hasn't caused any severe side effects. I know that's not true for everyone. The Avastin she's on now has been very successful in keeping the tumor at bay.
The link to receive a consultation at Duke, either in person or over the phone, is: https://tischbraintumorcenter.duke.edu/adult-patient-self-re
The phone number is: 919-684-5301.
Best of luck. Your brother and you are in my thoughts.