I admire the old man's determination and dedication to do what he feels is his duty. Those are admirable traits!
The problem is that somebody is taking advantage of that man's courageous spirit. I cannot condone how the teachings, writings, and actions of the Watchtower Society use guilt and shame to induce him and others like him to do more than he is physically capable of. "We must strive harder in the field work, the goal is very near, there is so little time left!" There is a huge difference between encouragement and exploitation.
Imagine a High School coach who yells at his players that they're not working hard enough, that they aren't doing all they should to help the team, that they should stay out in the hot sun practiclng far past the point of exhaustion--then a player drops dead of heat stroke. What do you think of a "leader" pushing his charges so hard? Such behavior is inexcusable! It is loathsome! It is sinful!
That is how I see the Watchtower behaving when they push for more, more, and even more field service. That tired old man has already given you more than his fair share. Instead of asking to do more than his body allows, why not show him appreciation for what he has already done and allow him his well-earned rest?