This is because "a"/"an" mean "one". The stress generally is on it meaning "[any...

archievillain 36 days ago | parent | context | favorite | on: English: A vs. An

This is because "a"/"an" mean "one". The stress generally is on it meaning "[any] one", but in your example "a" is used for its numerical meaning instead; the sentences "I have a son" and "I have one son" are interchangeable in this instance.