Goku Jr is the great great son of Goku, I don't know who is the father just that he has blood of a Saiyan. In a very young age and without training a lot reaches the Super Saiyan level. Is grandmother Pam said that he is just like Goku that's why she ask that the put that name to him and also said that in him there is a hiding power. He later fights Vegeta Jr in the tournament and no one knows who won. Oh yeah he meet Goku using the 4 star Dragonball. The story ends there so you can imaged what's happend then, but one thing I due know is that, he went in search for the Dragonballs and a new series started from there; who know may be.