John, This is such sad and distressing news about your brother. Especially after having gone through the removal of his prostate, only to discover he now has another form of cancer in the same year. You’re wondering how this new cancer developed so fast.
Most likely this didn’t happen quickly. There’s the possibility that your bother’s prostate cancer metastasized. This allows cancer cells to spread to his lymph nodes and take off from there. It wasn’t until the cancerous cells reached a certain level of development that the symptoms became apparent.

I found a very informational article from MedicineNet that might help you understand what may have happened with your brother.
~ How Quickly Does Prostate Cancer Spread?
https://www.medicinenet.com/how_quickly_does_prostate_cancer_spread/article.htm
My heart goes out to your entire family at this difficult time. I’ve been through similar with family members and what I’ve learned is that hospice will keep your brother comfortable. But it’s equally important for family to spend time with your bother, share stories and memories to keep his spirits up. Also treat him conversations beyond his cancer diagnosis. Not everything has to be morose and centered on his condition. Have tissues handy because there will be tears but there can also be laughter. Does your brother have a large family? Grandchildren?