User blog comment:Aegis Runestone/Hi, It's been a long time. Would anyone like to catch me up on AC?/@comment-2112031-20170529200159/@comment-2112031-20170531102151

Desmond's son was born in 2005, when Desmond was 18. In his recordings in AC4, Desmond mentioned how, through Clay's memories, he had seen how Juno, Minerva and Tinia (Jupiter) had used their calculation to see several possible outcomes for Desmond, one in which he married early and had a son, one in which he stayed single in New York, one in which he moved to San Francisco to be a waiter, one in which he worked at an auto-body shop in Chicago, one who never ran away from the Farm in the first place and on and on.

Evidently, Desmond conceived a son but remained unaware of him. The boy's ancestry and place of birth pretty much make it unambiguous that Desmond is his father, something that Aymar Azaïzia more or less confirmed during his Reddit AMA.