He wants to get married to a woman whom his parents rejected because of a physical defect in her

Praise be to Allah.

Firstly: 

A man should choose for himself a woman who is religiously committed and of good character, because the Prophet (blessings and peace of Allah be upon him) said: “Women may be married for four things: their wealth, their lineage, their beauty and their 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. 

He may marry a woman who is sick or has a disability or has a physical defect, because he has the right to choose and it is up to him. 

Secondly: 

A man should ask his parents’ permission to get married and consult them concerning the one to whom he wants to propose, and he should try to please them because of the great rights that they have over him. 

If he wants to marry a particular woman and his parents tell him not to marry her, then he should obey them so long as he does not fear that he will fall into haraam if he does not marry this particular woman. Ibn Salaah, al-Nawawi and Ibn Hilaal stated – as mentioned by al-‘Allaamah Muhammad Mawlood al-Mauritani in Nuzum al-Baroor – that it is obligatory to obey one’s father if he forbids his son to marry a particular woman. 

But if he fears falling into haraam with this woman, then warding off evil is to be given precedence over obeying his parents.  

If you are keen to marry this woman, then you should convince your parents of the reasons why you have chosen her. Then if they agree, praise be to Allah, but if they insist on refusing, then it is better for you to obey them. 

You should not let that upset you, for Allah has put a great deal of good and blessing in obedience to them (parents). It is no secret that the parents have great rights and virtues which call one to show gratitude towards them and to honour them and treat them kindly, and to sacrifice a great deal of pleasures for their sake. 

May Allah help us and you to do righteous deeds. 

And Allah knows best.