"然后他发现" can be translated into English as "Then he discovered" or "Later, he found out", depending on the context and the tone you want to convey.
"Then he discovered" implies a sense of surprise or new insight at a subsequent moment.
"Later, he found out" suggests that he learned the information at a point in time after an initial event or situation.
Both phrases are commonly used in English to convey the idea of someone realizing or uncovering something after a certain period or event.