Hoping for the best but it doesn't look good. That is rugged country.
My mind always jumps to bears but getting stabbed by a branch during a fall and bleeding out, breaking a leg, or even just having a heart attack/stroke are more likely than getting killed by a bear. At least that is what I tell myself when I go out alone in bear country.
It's usually not good to speculate, but this sounds like it could have been a heart attack. A mile uphill from the truck is about the right distance that he would have been exerting himself long enough that something could have happened.
A guy that I went to church with that seemed to be in good shape died from a heart attack a few years back going for a hike about the same distance up the trail.
Didn't Montana have a big fitness program for all state employees because they had found a correlation between heart attacks and the start of hunting season?
As someone who is approaching that age I think about how shocking it is and what a shame to die so young. On a personal note though, I hope to spend my last minutes in the woods or along the rivers doing what I love most and not in the nursing home. God Speed friend.