I'd agree that cancer is most likely, especially given his relatively young age. Acute issues (like a stroke or appendicitis) wouldn't really get called an "illness" at all, and most of the other terminal diseases you expect to see first diagnosed in people in their 60's (like early onset Alzheimer's, Parkinsons, ALS, etc.) don't typically have much of a mortality rate until people reach their 70's. Plus, cancer seems the most likely to fly under the radar until relatively late, given how many cases have few or no symptoms until after the tumor gets metastasized or inoperable.

Exceptions exist of course, but cancer seems to make the most sense to me.