Was wondering about this boy myself. It could just be the form of Guts and Casca's child of how it would have been if it hadn't been tainted by Femto. With the rebirth of Griff, it seems dimensions and realities are overlapping and mixing more. Technically, Guts and Casca's child "died" when Femto tainted the foetus. With Griffit's rebirth, the mixing of the dimensions etc, the form of the cild that never was, appeared. maybe it got split and reborn in the egg apostle too, maybe it was able to form at the full moon on the beach since at full moon is when the power of magic is strongest, etc. This could be an explaination. I don't think it is Griffith tho. 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.