What is the ruling on working in a job, the salary earned from it, and the possessions bought with this salary, if the father sought to obtain it through favoritism (wasta) without the son's knowledge, even though the son succeeded in all the tests?

There is no sin upon the questioner in accepting the job as long as he succeeded through his own efforts and not through his father's intercession, until proven otherwise with evidence. If it is proven that his success was through intercession, this does not mean that the job is unlawful if he is competent for it and no injustice resulted from the intercession to anyone. There is no sin upon him in benefiting from the salary if he performs his work diligently and as required.