Is Scar Nuka's father?

No.

According to Flip Kobler, Nuka was originally going to be Scar's son alongside Kovu, and Vitani. In that script, Zira was Scar's mate so that their cubs would have a claim to the throne. However, Kovu was downgraded to the protege of Scar, whilst Kobler had no say in the matter regarding Nuka and Vitani's relationship with Scar. But Darrell Rooney had stated on Facebook that Zira and Scar's relationship was intentionally censored to make her "just a follower," which would likely imply Nuka is unrelated to Scar.

No Nuka never calls Scar father neither does Vitani o Kovu. Zira was just a VERY loyal follower to Scar(Yes a deleted scene does describe them as mates, but no one cares about that)