This I got this wrong, too, but the explanation is here: https://en.wikipedia.org/wiki/Hawaiian_grammar#Gender . Might not be all that helpful at first, but here we are, starting to get into the really challenging parts of Hawai'ian grammar.
Aloha~! This is a question between a-class vs. o-class possessives (search more on this when you have time). In Hawaiian thought, having a child is something that is acquired through life, and you are not inalienably born with. For that reason, it should always be "kāna" vs. "kona". Contrast this with "his mother." This would always be "kona makuahine" as his mother has been his mother since he was born into life.
Hope this helps!!