What is the ruling regarding my continued employment with a person when it became clear to me that he did not fully pay one of the workers his due, and am I thereby considered to be assisting the oppressor in his oppression?

If an employer wrongs one of his employees, this does not affect the lawfulness of his wealth or your work with him. Your permissible work with him is not a form of aiding him in his wrongdoing. There is no blame upon you for working with him, and it is recommended to advise him to free himself from injustices.