It occurred to me a few days ago that the son of one of my main characters should have died about seventy pages ago.

Violently.

That's not a bad thing to realize at this point in the process, and one benefit from having tried this novel-writing thing once before (for my MFA thesis) is that I know to sit on the idea, and keep writing the story as it stands (without killing him, and even including him in new material), until I'm utterly convinced such a thing is required or until the first major revision.

You can't stop working until you make up your mind, or write from this point forward as if what you're imagining has already happened. Even going back seventy pages and weaving the new event into the draft from that point doesn't seem like a productive course. Best to let it stew, get second and third opinions (by finishing the draft as conceived), in order to discover all the implications. I guess my primary change in approach is to avoid beginning any new, significant plot turns around this potentially doomed figure. But none were planned anyway; in fact he rather fades from view, probably because he was supposed to have died.