I think your idea of the moonlight boy is a good one. We all believe that he is the child of Guts and Casca. There is one one other aspect to it, I won’t go into because it’s too early in the story. What I think is hilarious is that no one in the squad really knows Guts! The two that have a vague idea of his background are Scheirke, because Flora told her he survived the eclipse along with Casca, and Puck! Puck unfortunately has the memory of a goldfish. He may be able to empathize and see pieces of what you are thinking, but it doesn’t last. There are secrets being kept in this little family and it’s not just that Guts and Casca have a child. No one knows this, they suspect the two have some sort of relationship but don’t know what it is. Serpico has his own secret that he has kept from Farnese as well, that he is her older brother. Ooof! Those are some big secrets!