As an idiom, the closest thing I can think of is to drop a bombshell:
: to surprise everyone
// She dropped a bombshell when she said she wouldn't run for reelection.
In the sentence in the question, the result would be as follows:
A doctor is dropping a bombshell on his patient.
While this doesn't explicitly say the news is that the patient will die, it would be implied by the use of the idiom and the nature of a doctor having surprising news.
But this also has to do with context. If the patient had already been told they were going to die, and the same thing were said, it's likely it would imply something else. Either that the patient were miraculously cured and wasn't going to die after all, or that (perhaps), things were much worse than expected and their death would happen sooner than thought.
Dropping a bombshell doesn't always imply something negative—but it often does.