Technically, Guts and Casca's child "died" when Femto tainted the foetus.
No, I think it's clear that isn't the case. Technically, it was corrupted by Femto, as explained in the manga.
The boy was in the harbor amonst the captured children who were about to be sent off as slaves and I think he went with Sonia to Griffiths camp along with the other children.
I imagine you mistook a random Kushan kid for the boy?
Anyway, refreshing my thoughts on this subject, I still think he's some incarnation of Guts' and Casca's child manifested and projected via Griffith's power. Aside from that, the real question is whether it's a passive occurrence or not on either the child's or Griffith's part.
Well that remains the most logical explanation to me as well even after all that time. Where else could the boy's "superior" powers come from? And we know a part of him still exists within Griffith too, strong enough to make him act without thinking.
As for whether it occurs passively or not... I believe that it is an occurrence related to the full moon. Simply said, when the moon is full and magic strengthens, the boy can muster enough power to manifest himself into the world. The question this raises though is how it is done. Does he appear out of thin air? Does he use Griffith's body? The second possibility seems a little unlikely as it would have to include a way to neutralize the "evil" power Griffith inherently possesses and that Schierke or Guts and Casca's Brands would have picked up (although if he could actually transform his body, I guess anything else would be possible).
A plausible way events happened that night: the moon is full, Griffith is asleep or in a transe. Zodd stands guard not too far away. The child appears one way or another, and starts travelling towards his parents that he can feel are nearby. Zodd picks up on this, and decides to follow the boy. Having seen who he's meeting, he turns off and goes back to the camp, keeping the information to himself.
