If Nuka is Scar's son, then, why does he call his dad by his formal name? Everything from the movie to deleted scenes has him refer to Scar as simply "Scar". I'd know the line, "Kovu, Kovu, Kovu. Scar wasn't even his father. He just took him", perhaps points that Nuka is Scar's son, but it seems obviously written to explain Kovu was not Scar's son.
It is true that Nuka was designed as more beefy and muscular. There are early concept artworks that prove this. It was also nice of your friend to find him on Facebook, but even with
The Lion Guard, it seems the relationship between Zira and Scar is still left to interpretation. Zira states they "were close" although she does not elaborate on what that means. They could be mates according to some fans, but to others, it's not. Its open-endness makes any following debate more interesting.
Strangely, it seems everyone involved during the making of
Simba's Pride has their own individual answer. Mr. Kobler and
Bill Motz, a storyboard artist, claim Nuka (and Kovu) were Scar's cub while the director, Darrell Rooney, states Zira's relationship with Scar
"was left obscured to make her 'just a follower'". It's confusing, but I'd stick with Rooney's statement that Zira was a devoted follower and the father of her children is not Scar.