It was clearly stated in the film.

i heard somewhere that Scar and Zira had Nuka and Vitani. Then Zira and an uknown lion had Kovu.

I realy think he is! Even though Nuka told other wise. I mean look at kovu and scar together! they look very simillar. And do you think that just some rogue lion could make kovu look so simillar to Scar ? I don't think so!

Kovu said himself that Scar isn't his father..

Kovu is not Scar's son for sure. Kovu said so himself same with Nuka.

Even though they look similar we have to keep in mind they are still lion king characters, and the animation for the second film was lower budget than the first so they obviously just took the characters and modified them to look like their own character. I mean just look at the outsiders they all look super similar to Vitani and zira but they aren't all zira's children that's just how they drew the characters Allthough they should have put more thought and care into their design.

It is spoken by Kovu (and Nuka) that scar is not Kovu's father.