The word that fits is "mustn't". It indicates that a logical deduction is being made: If he wanted the job he would be here; He isn't here; Therefore he does not want the job.
The word "mustn't" is not a stronger version of "shouldn't" in this case, because it doesn't indicate that the absentee is forbidden from wanting the job. Rather, it is a stronger version of "probably doesn't", like "certainly doesn't". It must be true that he does not want the job, because that follows logically from his actions.
This is the 4th definition of must here in Merriam-Webester:
4 : be logically inferred (see INFER sense 1) or supposed to,
e.g., "it must be time"