No one can tell you my lovely if he's going to die, personally I would be more worried about metastasis where it's in multiple locations. But only really an oncologist could really say.
It's strange how it plays out, I knew a woman riddled with cancer, you wouldn't have though anything was wrong, even though it was terminal. Another person who had quite an innocuous and prevalent cancer, (protrate) who ended up somehow having it spread to another sole organ, we thought it was just random pain but the decline was quick.
How much pain is your FIL in? Is he on palliative care? One thing to maybe prepare for is that a family member will need to be there, they have good pain meds that can be delivered by a syringe driver, then the relative gives PRN doses.
Radiation is a bit like sunburn, but you can also get nausea/vomiting and other symptoms. It's pretty much an individual thing, but I'd say you need to mitigate for the fact he won't want to sit 'on' where the radiation has been applied. So not knowing about the specifics, I'm thinking maybe a rubber ring cushion like they have for people with bowel issues, plus a bolster high up his back aimed at the thoracic spine (ribs) to push pressure away from the bottom.
With that area of the spine you're possibly looking at continence issues, so bear that in mind, it can be sudden, unlike the other woman who made her Mum clean up her wee, there's continence aids, but guessing you'll have to adapt them so they avoid where the radiation is going. I say this as a possibility, but if it does start happening, don't make a big deal, just try and help how you can.
I wouldn't like to say oh it sounds like he's ok and could beat this, when I don't know the full picture. I sincerely hope he does.
I'm just sorry your family is in this situation. 