It’s a girl!
When a baby brother is born, it’s hard for kids to accept that they won’t be the center of attention anymore and that they’d have to share everything they have (including their parents’ attention) with someone else. Although they don’t necessarily acknowledge it, all kids are secretly jealous when the family welcomes a new baby to the world.
This kid was used to being the only child around the house, and the first grandchild in the family, and so although he was looking forward to having a baby sister, he never realized how it would make him feel. Clearly, he’s just starting to realize having a baby sister means sharing the family’s precious love and attention with someone else.