About his name and surname, it’s necessary to mention the following points:
Name implies proper name or proper noun that is kept for an individual at time of birth or at time of manufacture without keeping in mind its meaning.
Family name or surname is a recent custom from France. Under the term Fumille, (in English, Family) household, kin, near ones, relatives & family.
In Iran, family name or surname has become customary only in recent half-century.
The lexicon of holy book of Mr Hoax, page 59:
It had become the practice of the Jews to keep names by considering its meaning, considering the person’s character, or considering the events at the time of birth. During birth or circumcision of the infant, either the mother or father or both would consult friends and then name their infant accordingly…..Sometimes, inspired by prophetic names and meanings, names were selected…..and at times, due to the above reason, someone’s name would be changed during his lifetime. For example such names as Abraham, Saraa and Jacob. Sometimes the name is derived from the special event that occurred during his life-time like Samuel, Jesus, Jacob, Moses etc. Many a time, they would select divine names. For instance, they would add to a name such words as El, Yah, and Ya hoo etc and select such names as Samuel, Ya hoosh’a and Advinah. At times, one sentence was selected as name.
As per religious creed, the names of outstanding figures, such as the prophets, have been made by way of inspiration or revelation.
Among the Arab tribes, three types of names were common:
Title (Laqab): comprising of a meaning that denotes some perfection.
Agnomen (Kunyah): used with such terms as اب and ام and such words as اخ، اخت، ابن، بنت (which mean the father of, the other of, the son of, the daughter of) like (ابو القاسم) Abul-Qasim and (ام سلمه) Umma-Salamah.
Regarding the Shia holy Imams, name title and agnomen were used by inspiration, revelation and divine command and each one selected for specific reason. Traditions from the holy Prophet (s.a.w.a.) support this view.
About Hazrat Mahdi (a.t.f.s.), we shall set forth traditions that show his names, titles, and agnomens:
Imam Hasan Askari (a.s.) said to his wife (Hazrat Narjis Khatoon):“Soon you shall carry a son whose name is Muhammad and he is the Qaem after me.” 
In 256 A.H, Imam Hasan Askari (a.s.) was honoured with a son and he named him “م ح م د ” (Muhammad)
Abu-Ghanem, the servant, says:“Hazrat Abu-Muhammad Hasan Askari (a.s.) was honoured with a son, and he named him Muhammad. On the third day, he showed his son to his companions and said, ‘he is your master after me, and my successor…” 
Abu-Haroon says:“I saw Saheb-uz-Zaman (a.t.f.s.) step into this world on a Friday 256 A.H..” 
Mahdi (a.s.) was born on Friday…His mother is Raihaneh, who is also called as Narjis, Seeqal and Susan…
“Al-khalaf-us-Saleh” was born.
Hasan-ibn-Husain Alawi says:“I approached Abu-Muhammad Hasan-ibn-Ali (a.s.) in Samerra and congratulated him for the birth of his son “Al-Qa’em” .
He was given the title of“Al-hujjah” ,“Al-khalaf-us-Saleh” and“Al-Muntazar” .
Name, title, agnomen:
Hazrat Jafar-ibn-Muhammad (a.s.)said: “Khalaf-us-Saleh is from my progeny. He is the Mahdi (the one guided). His name is
“م ح م د ” His agnomen is Abul-Qasim. He shall rise during the end of time.”
Hazrat’s visage and appearance:
The holy Messenger (s.a.w.a.) said:
“Among all the people, he resembles me the most in creation.”
“He looks like me and his way is the same as my way.”
“Mahdi is from my offspring. He is like a sparkling star; his colour is the colour of the Arabs, and his body as the body of the Israelite.” 
About his description, we quote from“Khorshid-e-Maghreb” , page 28 as follows:
“His face is tawny; his eyebrows crescent-shaped and elongated; his eyes big, black and attractive. His shoulders are broad, his teeth shining and wide. He possesses a narrow and beautiful nose. His forehead is broad and bright; his bones strong like a rock, and his hands and fingers magnified. His cheeks display less flesh and are slightly yellowish (due to sleepless nights). A black spot appears on his right cheek. His muscles are curved and strong; his hair curls over his ears. His limbs are perfect and beautiful; his figure is good-looking and charming; his face covered with halo of modesty and drowned with splendour. His appearance gushes with splendorous leadership; his looks metamorphic; his cry roaring and breath-catching.”