Marriage of one who is mentally deficient

I have a brother who is 30 years old and wants to get married, but he has a problem which I will sum up as follows. He is a regular person, with a good memory, his body is healthy and he looks fine; he can tell men and women apart and he understands when we speak about matters of marriage and so on, but he cannot distinguish matters in the sense that he does not understand the meanings of marriage and divorce, or the rights and duties of marriage and so on. My question is, it is permissible for him to get married or not, knowing that he says “I want to get married”? And Allaah is the source of strength.

Praise be to Allaah.  

It is permissible for him to get married, but his situation must be explained clearly to the guardian of his wife and the wife must be told about the deficiencies in his mental understanding and ability to discern matters, and that he does not understand the meanings of marriage and divorce, and the rights and duties of marriage, and that he does not know how to pray properly, and that he does not have a job and does not have any qualifications, and that he does not have enough knowledge to know what will benefit him or what will harm him. Then if he does get married, his brother or his father will have to supervise him and take care of his needs, and ensure that he has accommodation and money to spend, and other things that are needed for marriage such as the mahr (dowry) and the like, because this deficiency is regarded as a fault for which the marriage may be annulled. But if his situation is explained clearly to the woman and her guardian, then the Muslims are bound by their conditions.