Several doctor visits concluded that it was the long hours and insane amount of stress that was severely crushing his immune system.
Moral of the story is love what you do and take care of yourself: nothing is as important as your own health and happiness
In stressful periods, it's likely not stress crushing the immune system, it's the indirect relationship that stress causes bad quality sleep and low amount of sleep, that in turn crushes the immune system.
If, even if under stress, you manage to work out a system/habit that allows you to get proper sleep, you'd likely be ok.
To me that's the real question. I think either option is defensible, depending on what one values in life. I've known certain people who pulled 80 hour work weeks for years only to give away double-digit percentages of their salaries to charity, because that was what was important to them, and I don't think they were wrong for doing that, just making choices at a margin I would find intolerable far earlier.
He may need his tonsils out. I got mine out in my 30s. It was 3 painful weeks to recover but my quality of life is a lot higher.