He is Jake Long's son who appears in a flash-forward shown at the beginning and the very end of the episode "Bite Father, Bite Son". He's very similar in appearance to his father, although his face is thinner and his hair is wilder with only three green stripes, on the sides and on the top of his head.
"Bite Father Bite Son" (only appearance)
Although the question of who this character's mother is was never confirmed, it was presumably Rose.
Jake Long Junior with Jake Long Senior
Jake Long Junior walking with his dad Jake Long Senior.