Starting to really flesh out my tlk cannon surrounding Kivuli and this one might take a moment to explain XD

so!

In my cannon, scar mated all his lionesses and fed the males to his hyenas to prevent rival heirs growing up in his pride.

Jua is the only male cub as he was born after Simbas return and after the other children of Scar had been exiled (not all got to live within Ziras pride- Kivuli was one of those left homeless)

His birth was a secret but Simba found him and tried to get rid of him, gashing his face and making him flee his home.

He bumped into Kivuli, who recognised his Scar like features and adopted him as his big half sibling.

Since she never saw a male cub who wasnt Nuka and Kovu, she became instantly attached and protective of her adopted son.

EDIT

so he is the cub of Scar and Sarafina.
After Nala left, Sarafina was weak willed enough for Scar to have his way with her.

she birthed him in simbas early reign