I'll chime in and agree with the others here, some good advice and words of wisdom

I am impressed that he is 90; that's the part I'd focus on if I were you. Most of my relatives passed away in their 60s and 70s.
You are armed with information he never had, there is a lot more you can do these days (listed above) to help yourself.
These things are highly variable. Spondy runs on my mother's side of the family. My great grandfather's stomach inflammation was so great he ended up with stomach cancer; today I have prilosec, and now Humira. My aunt has had arthritis problems since her teen years and just got dx'ed with psoriatic arthritis in her 60s, 50 years of suffering first. And you know my story. And my mom has it much more milder.
So, I wouldn't worry by looking at your grandfather if I were you. Take one day at a time and just focus on you
