Hengist was from Jütland, he was not born in Saxony, Germany. Therefore he did not have a Saxon son. His only son is attested in the Foundation for Medieval Genealogy as Oisc, aka Oerc, aka Aec, the founder of the Oiscingas. Hengist and his brother Horsa were Jütish commanders invited by Vortigern to fight the Picts and Celts in England. They founded the Kingdom of Kent which remained in control of the Jütes for quite some time.
The Saxon Kingdoms had one defining characteristic, they ended in "sex" as in Sussex, Wessex, Essex, etc. https://www.history.org.uk/primary/resource/3865/anglo-saxons-a-bri...