Is the father considered unjust to one of his children who refuses to marry and to accept his father's help, while demanding large sums of money, at a time when the other children need the father's help for their education and preparation for marriage?

The question does not indicate that the father has wronged his son. A father is not obligated to financially support his adult son who is capable of earning a living. Even if maintenance were due and lapsed with the passage of time, the father is not required to compensate for it unless ordered by a ruler. The father must assist his son who needs financial support for marriage in a reasonable manner, and he is not required to transfer ownership of an apartment to him; it is sufficient for him to allow him to reside in it. For a son to sever ties with his father is forbidden and an injustice. The son must repent, apologize to his father, show him dutifulness, and treat him with kindness.