Which version is idiomatic? If more than one version is idiomatic, explain the difference please. I've tried to explain why each sentence can/can't be used. I need a native speaker's view on that.
He is in hospital. He will have an operation tomorrow. <--- Can't Future Simple be used here because the man has the arrangement with hospital about that operation?
He is in hospital. He's going to have an operation tomorrow. (NGram) <--- Present Continuous because of the arrangement with hospital
He is in hospital. He has an operation tomorrow. <--- Present Simple because of timetable on that operation