It is customary when praying for an ill patient, Heaven forbid, to mention his motherís name, for example, "Abraham Ben Sara." One reason given to explain this custom is that when we pray, we need to be as precise as possible, and a personís relationship to his or her mother is more certain than the relationship to a father. Since there were people witnessing the birth who can definitively ascertain that a person is the motherís child, whereas no such definitive claim can be made determining a personís father, calling somebody by the motherís name is a more precise form of prayer. Indeed, the Zohar (Parashat Lech-Lecha) notes that when King David prayed, he called himself "Ben Amatecha" Ė "son of Your maidservant" Ė rather than "son of Yishai," because his relationship to his mother was more certain.
What name should one use if he does not know the patientís motherís name?
The work Mehkereh Aretz (Yoreh Deía 26) writes that in such a case, one should use the fatherís name. The Kolbo (141) indicates that the fatherís name should always be used when praying for an ill patient, and thus if the motherís name is not known, we may rely on the Kolboís view and use the fatherís name. If neither parentís name is known, then the patient is called "So-and-so Ben Hava," as we are all considered the children of Hava, the mother of all mankind.
Summary: When praying for an ill patient, he is referred to by his name and his motherís name. If his motherís name is not known, the fatherís name should be used, and if neither parentís name is known, then the patient is called "So-and-so Ben Hava."