I thought the Accusatıve ending was only for Direct Objects. This would be a Predicate Nominative in the Nomınatıve case. I didn't know there was a difference here.
Thank you AlexinNotTurkey, below! - I got confused again since they look similar. I really need to practice those possessive endings some more! So the difference has to do with the noun not getting the possessive ending in "this job", but the noun does get the possessive ending in "his job"
"It is his job." = "O, onun işi."
"Bu iş onun." = "This job is his."
Boy, I really need to be careful! For a bit the possessive ending looked like the accusative to me. I guess the real thing to notice is that the possessive ending is not used on the noun in "this job" but it is used In "his job".