Helen, you could say" Overall he's a very calm man." or Usually he's ... or He's usually an even-tempered man... or you could use he's always, choose one -

Both those phrases are fine but if you want to make it clear that you are referring to how he reacts in difficult situations (instead of just saying that he is a calm person in general), a native speaker would probably say something like "He's always calm in a crisis."

He is a person of calmness.