It's passivity about things you can't control. I'm all for engaging with things within my control. I just spent the morning under my car replacing CV axles because I wanted it done faster, for less, on my own schedule. Why sit around waiting? I suppose what I'm advocating here isn't passivity at all, but engaging in loving his partner in ways that would matter to her while he still can. That can be a very active and engaged way of handling this.
After watching several family members die from cancer though, the notion of fighting that rather than being with my family as they pass sounds terrible. At some point you have to accept that death is on the way. Love people while they're here. Curing cancer is a generational goal accomplished by huge numbers of people, hopefully, not a single person in short order.
I have very positive thoughts about his love and concern, too. I just hope it's put to good use while it still can be.
Also, he reminds me of my dad who was similarly convinced if he cared for my mom just right she'd be okay. It has been almost 20 years and he still hates himself for it. There was no stopping it. There's no sense in having these expectations of yourself.