I am currently bored of supporting Kopa as cannon, or at least him being the Simba's cub and the cub at the presentation in the first movie. This is what I now think:
Kopa wishes to be the son of Simba and Nala because he greatly respects and loves the king and his legacy, much like a boy who loves baseball and has a favorite star in the game.
I think the parts where he and Simba and Nala are communicating are in his dreams. He tells his friends from he is the king's son and tells tales of his strength and glory. I think his friends are entertained by this and are not really his friends just associates, and others dismiss his wishful thinking and are friends with him because they are acceptive of him. I believe he lives in a pride where his parents don't dedicate as much attention to him, so he feels just in dreaming of the King and Queen as his mother and father. So...
My theory means Kopa is NOT Simba or Nala's son.
My theory indicates that Kopa is in the time before Kiara's birth, but close to it.
Whatcha think?