Shimon isn’t entitled to a job with Reuven, or if he has a job he isn’t entitled to perpetual employment with Reuven.
Reuven doesn’t need cause to discontinue employing Shimon. Employment is “at will”. Reuven can one day wake up and decide he no longer wants to pay someone to fill the position Shimon is currently filling. Or he can wake up one day and decide he wants to give the job to his son rather than continue Shimon in it. This is fully within the scope of halacha (and incidentally secular law.)
Shimon isn’t entitled to have Reuven pay him “to make as much money as when he was in his desired field.”
1] Shimon has no case against Reuven in Beis Din.
2] Reuven didn’t wrong Shimon to be liable in Beis Din shel Maaleh or Beis Din shel Mateh.
3] Reuven never caused Shimon damages and thus there are no damages due.