He is practicing ‘azl with his wife so that she will not get pregnant because she is still studying
Question

                Is it permissible to practise ‘azl (withdrawal or coitus interruptus) or a similar method (of contraception) if that will go on for two years or more, so that the wife can finish her studies before she gets pregnant? Or does this go against one of the most important reasons for marriage in Islam?.
Praise be to Allah.Firstly: 
One of the purposes of marriage in Islam is to have children and increase the numbers of the ummah. 
Abu Dawood (2050) narrated that Ma’qil ibn Yasaar (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) said:  “Marry the one who is loving and fertile, for I will be proud of your great numbers before the nations [i.e., on the Day of Resurrection].” Classed as saheeh by al-Albaani in Saheeh Abi Dawood, 1805. 
Secondly: 
Practising ‘azl – which means ejaculating outside the vagina – is permissible, but that is subject to the condition that it be with the wife’s permission, because one of her rights is full enjoyment, and another of her rights is to have children, both of which are lost as a result of ‘azl. 
It was narrated that Jaabir ibn ‘Abd-Allaah said: We used to practise ‘azl at the time of the Messenger of Allaah (peace and blessings of Allaah be upon him) when the Qur’aan was being revealed. 
Narrated by al-Bukhaari, 4911; Muslim, 1440. Muslim added: Sufyaan said: If there had been anything wrong with it, the Qur’aan would have forbidden it.   
Shaykh al-Islam Ibn Taymiyah said: 
With regard to ‘azl, some of the scholars regarded it as haraam, but the view of the four imams is that it is permissible with the wife’s permission. And Allaah knows best. 
Majmoo’ al-Fataawa, 32/110 
See also Question no. 11885. 
Thirdly: 
It is permissible for the spouses to agree on family planning so long as that is temporary and not a permanent thing – subject to the condition that the means used does not cause any harm to the woman. 
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: 
With regard to using temporary means of preventing pregnancy, such as when a woman gets pregnant easily and pregnancy is exhausting for her, and she wants to space her pregnancies so they will be two years apart and so on, this is permissible so long as her husband gives his permission and so long as that will not cause her any harm. 
Risaalat al-Dimaa’ al-Tabee’iyyah li’l-Nisa’  
And Allaah knows best.