6. He's been getting bad headaches since he has been in the army.
7. He's been getting bad headaches since he was in the army.
8. He's been getting bad headaches since having been in the army.
9. He's been getting bad headaches since being in the army.
According to A Comprehensive Grammar of the English Language, by Randolph
Quirk et al., page 539: In #8, an unusual form (my bold), it would be implied
that he was no longer in the army. In #9, he might or might not be still in
the army.
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