Read it in the context of someone who would now be old enough to have died through a variety of causes, be it a motorcycle accident, cancer, or a heart attack. You also now have to read it in the context of (spoiler alert) the author being dead as well.

You don't, because it has no bearing on subsequent events. It might be interesting to consider that on a subsequent reading (ZAMM certainly stands up to multiple readings) but you're just as apt to mislead yourself by projecting your incomplete knowledge of his eventual end onto his behavior and utterances in the book; so you might find yourself reading it and thinking 'oh, this element in his character no doubt played into his murder' when iirc the circumstances of his death were essentially random.

I read the book about 5 years back, it interested me so much, and I ofcourse wanted to research the father/son's present day life. Well it was heart breaking. So I get what you are saying.