Should he get married without his father’s approval?

A son is not doing wrong if he chooses a woman who is religiously committed and of good character, for this is the advice of the Prophet (peace and blessings of Allaah be upon him) with regard to marriage. It was narrated from Abu Hurayrah (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “A woman may be married for four things: her wealth, her lineage, her beauty or her religious commitment. Choose the one who is religiously committed, may your hands be rubbed with dust [i.e., may you prosper]!” 

(Narrated by al-Bukhaari, 4802; Muslim, 1466) 

There follows some advice for you and your father from Shaykh Ibn ‘Uthaymeen, that has to do with your situation. 

The Shaykh (may Allaah have mercy on him) said: 

The question leads us to offer you two points of advice. The first point is addressed to your father, if he insists on not allowing you to marry this woman whom you describe as being of good character and religiously-committed. What he should do is to give you permission to marry her, unless he has a legitimate shar’i reason that he knows and can explain to you so that you will be convinced and your mind will be put at rest. He should weigh up this matter himself: if his father had refused to let him marry a woman whose religious commitment and character he admired, would he not have thought that this was wrong and a suppression of his freedom? If he would not like his father to do this to him, then how can he let himself do the same to his son? The Prophet (peace and blessings of Allaah be upon him) said: “None of you truly believes until he loves for his brother what he loves for himself.” 

It is not permissible for your father to prevent you marrying this woman for no legitimate shar’i reason. If there is a legitimate shar’i reason then he should explain it to you so that you will understand. 

With regard to the advice which we give you, we say that if you can forget about this woman and marry another, thus pleasing your father and avoiding a split (between you and your father), then do that. 

If you cannot do that, because you are emotionally attached to her and you are also afraid that if you propose marriage to another woman that your father may also prevent you from marrying her – because some people may have envy or jealousy in their hearts even towards their children, so they do not let them have what they want – I say that if this is the case and you cannot be patient and forget about this woman to whom you feel emotionally attached, then there is no sin on you if you marry her, even if your father objects. Perhaps after you get married he will become convinced and the feelings in his heart will go away. We ask Allaah to enable you to do that which is in the interests of both. 

Fataawa Islamiyyah, 4/193-194 

And Allaah knows best.