Why it's the son
Many fans debated, whether Simba's cub from the original TLK is male or female (i.e. can be Kiara from the sequel).
In early drafts of the original film, "King of the jungle" a.k.a. "King of beasts" by T.J. Allen and Ron Bass, Simba's and Nala's cub is a boy:
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In the workprint of the final film, Pumba (the dialogue on 3:35-3:38) refers to the lion cub as "little guy", not "girl", Simba and Nala don't say the opposite (as well as Rafiki) to Pumba, i.e. the cub is a boy indeed: https://youtu.be/aYeFyo-X4-Q
Sure, many can say that in "The making of" the creators said that they never stated the cub's gender, but I think that the production materials perfectly show the creators' real intentions and ideas.
P.S.In the novelization of the remake the cub is also described as a male. But it's another story and universe..













![Nirmala Avatar [F2U] Nirmala Avatar [F2U]](/sites_content/lion_king/upload/fanimage/kami/kami147fp.jpg)







However, that being said, it does not mean that the original film's creators intended the cub at the end of the film to be Kopa (I say that because some people might think it to be so). Kopa is not canon to the movies. I believe the filmmakers simply intended it to be just a male cub (Not Kopa) until they decided to leave it genderless and just 'end' it there.