The son of Gorbonianus was a legendary king of the Britons as recounted by Geoffrey of Monmouth. He was a son of King Gorbonianus but was never given a name in the text.
According to Geoffrey, he was a good and prudent king who emulated his uncle and predecessor, Elidurus. He was just and compassionate to his people and never faltered in his righteousness to them. When he died, he was succeeded by his cousin, Marganus, the son of Archgallo.
References
Son of Gorbonianus Wikipedia(Text) CC BY-SA