He is suffering severe pain in his lower back; does that mean he should not get married?

You should refer your case to a specialist doctor. If it is proven that this kind of pain may affect fertility or prevent intercourse, or makes a person unable to work and earn money, then you have to inform the woman you want to marry about this. If she accepts it, then there is nothing wrong with you marrying her. If you do not point it out then you are deceiving her. The Prophet (peace and blessings of Allaah be upon him) said: “Whoever deceives is not one of us.” Narrated by Muslim, 102. 

What we have mentioned is based on the correct view, which is that every fault that may affect the purpose of marriage must be disclosed; the spouse has the right to annul the marriage if such a fault is discovered after being concealed. 

Ibn al-Qayyim (may Allaah have mercy on him) said: … by analogy, every fault that puts the other spouse off and hinders the purpose of marriage, namely compassion and love, should be disclosed and the option of annulment given. (Zaad al-Ma’aad, 5/166). 

And he said: Whoever studies the fatwas of the Sahaabah and the salaf will realize that they did not specify one fault (as grounds of annulment) to the exclusion of others. 

And he said: If the Prophet (peace and blessings of Allaah be upon him) forbade the seller to conceal any fault in his product, and he forbade anyone who knows of it to conceal it from the buyer, then what about faults that affect marriage? The Prophet (peace and blessings of Allaah be upon him) said to Faatimah bint Qays, when she consulted him about whether she should marry Mu’aawiyah or Abu’l-Jahm: “As for Mu’aawiyah, he is a poor man who has no money, and as for Abu’l-Jahm his stick is always over his shoulder [this may mean either that he travels a lot, or that he habitually beats his wives].” From this we know that disclosing faults with regard to marriage is more appropriate, so how can concealing them and deceiving people be a means of completing the marriage ?

From Zaad al-Ma’aad, 5/168 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said: The correct view is that a fault is anything that affects the purpose of marriage, and undoubtedly the purposes of marriage are intimacy, service and producing offspring. These are some of the most important purposes, and if there is anything that prevents this purposes being fulfilled, then it is a fault. Based on this, if the wife finds the husband to be sterile or the husband finds the wife to be barren, then this is a fault.

From al-Sharh al-Mumti’, 5/274, Markaz Fajr edition. 

And Allaah knows best.