Answer:
Nothing happened to him. He doesn't exist in The Lion King movie series but only in The Lion King: Six New Adventures. It is official by the first film's makers that the cub at the end of the first film is not Kopa. It wasn't given a gender or a real name in its own film as they say this on the commentary track on the DVD. And this results in its only official gender and name being the girl Kiara from the only official movie sequel. Thus, nothing happens to Kopa as he never existed in the movie universe.