What is the ruling of Islamic law and secular law on a father's rejection of his eldest son's request for the lion's share of the company's stocks, and the father's expulsion of the protesting son for demanding his dues?

The fatwa is that it is impermissible to prefer one child over others unless there is a legitimate شرعي reason, such as the child's need, illness, preoccupation with seeking knowledge, or his فسقه (moral corruption). If the father singled out one of his children for a valid purpose, obedience to him is obligatory. But if he did not have a valid purpose, obedience to him is not obligatory, because obedience is only in what is right (معروف). There is no harm in suing the father to claim one's right from the company if no other means are available, while noting that the father's injustice towards his children does not nullify his right to their dutifulness (بر).