Why had Asta's son never known his true name?

In the book Crispin: The Cross of Lead, the thirteen year-old protagonist has gone his whole life being called "Asta's Son." When his mother (Asta) dies and he is accused of theft, he must flee the village to avoid being killed. The young man seeks the help of a priest, who reveals to him that his real name is Crispin. Crispin has been called Asta's Son until now because people in the village knew him in relation to his mother-- they did not have much reason to know his given name. Even his mother could have referred to Crispin as, "my son," or by some other term in relation to herself. For Crispin to be known as Asta's Son rather than his given name is a mark of his parentage. He was born out of wedlock and never knew his father. To be referred to as only his mother's son is a sign of the social stigma placed on those born outside of marriage.