Jack seems to be real son of Whitebeard.
His body size, its proportions, way he seats on his ship while reading newspaper and fact that his chin is covered with some metal thing all point to that for me. Plus his hair seem to be blonde same as Whitebeards when he was young.
Seems A BIT obvious but it would be cool, as Ace was Rogers son and didnt ended up as bad to the core powerful pirate, Jack being Whitebeards evil son could work out for me. Luffy beating up somebody who also has potential to be future Pirate King has a ring to it but we will see how it will go.
you would think that he would kill weavil if that was true even if he did not like whitebeard that would piss him off plus kaido would like him killing a warlord as well.