Hi Wally1024,
I wish there was a way to give you a clearer picture. From what you describe, your brother is younger than average for this cancer and his doctors have given him an aggressive regimen of treatment to confront the cancer. These are positive indications, but this cancer can be unpredictable (like most things in life) so there is no way to know exactly what his future path will be. We do have long-term survivors for whom this cancer becomes like a chronic illness with ups and downs, but there are also cases where the cancer regrettably can move more quickly.
If your brother stops responding to one chemo, there are others that will be tried. There are other potential treatments in the arsenel, such as various radiation approaches, targeted treatments if he has been identified to have a genomic mutation involved in his cancer, and new treatments now in testing that are available by joining a clinical trial.
Regards, Mary