The mother was half human as I recall.
Alexander did look like a young adult by his teens on DS9.
They had a different (3rd) actor succeeding the puny Brian Bonsall on DS9 (he was only 10-12 during the time he played him). That last actor, Marc Worden, was 21 when he played him in DS9.
I remember laughing the first time I saw Andy Keaton as Worf’s son. I thought that was BAD casting. I didn’t even remember the first kid to play him (the subject of the thread), but he looked a bit like Bonsall, actually. In fact, the photo of him at the top of the thread I thought WAS Brian Bonsall.