Because Julius Caesar was the boy's father.
Cleopatra's son, that she claimed was from Julius Caesar was named Ptolemy XV. History knows him as Caesarion, because Caesar allowed Cleopatra to call him that, but only in Egypt.
Caesarian
If you mean Cleopatra's son, his name was Caesarion and she claimed he was fathered by Caesar.
Cleopatra became Julius Caesar's lover and had a son with him.
Caesar's son by Cleopatra was named Caesarion.
Ptolemy Caesar, born in 49 BCE, also known as Caesarion [Καισαρίων] "little Caesar" was the child of Cleopatra VII Philopator and Gaius Julius Caesar the famous Roman military and political leader who played a critical role in the transformation of the Roman Republic to the Roman Empire.
Cleopatra and Julius Caesar were supposed to have had a son, at least according to Cleopatra. His name was Ptolemy XV or Caesarion. He was killed after Octavian conquered Egypt.
Octavius was Caesar's adoptive son but he was actually his nephew.
No, because Julius Caesar had no sons. Cleopatra claimed that her son was fathered by Caesar, but Caesar had his doubts and never recognized him. Cleopatra's son was killed 14 years after Julius Caesar was murdered.
Caesarian was Julius Caesar's son by Cleopatra.
Cleopatra's son, that she claimed was by Julius Caesar, was Ptolemy XV, nicknamed Caesarion.
Julius Caesar had only one illegitimate son, Caesarion, whom he had with his liver, Cleopatra VII of Egypt.