Improve the given sentence if necessary. He was too conscientious in the discharge of his duties that he could not serve that exploiter for long.

The phrase ‘that he could not serve’ is grammatically correct. This is because other forms of verb doesn’t go with the blank and this form of verb is grammatically and contextually correct. Thus, the correct formation would be, ‘He was too conscientious in the discharge of his duties to serve that exploiter for long’.