You can say 他在部队呆过。
Other answers are good . My answer is short, so maybe it is easy for you to say .
He - 他
Used to be - 呆过（our difference is that we translate these words in different way)
In the army - 在部队
October 26, 2017
If he was a soldier, you can say
October 25, 2017
October 30, 2017
This translation below is a bit more formal:
他以前在部队服役。( He served in the army.)
October 28, 2017
October 27, 2017
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