1)He is now on duty.
Is it possible to use on job or on work instead?
No. We say "at work", not "on work". "on a/the job" is possible ("on the job" has a vulgar meaning too).
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No. We say "at work", not "on work". "on a/the job" is possible ("on the job" has a vulgar meaning too). Neither means quite the same as "on duty".