I can imagine in Henry’s mind he’s been calling David ‘grandpa’ ever since he figured out who David was. It’s probably the relationship he needs most of all since he’s never had a true male figure in his life.
Henry has known, ever since seeing the coma patient, who he was. And yet other then David walking him home from MM’s party they’ve had very little interaction. He has a relationship with him bio-mom (Emma) and his his grandma (even if Snow doesn’t know they’re related) because she’s his teacher, the one who gives him the book, and his mom’s roommate.
But ‘Grandpa?’ is his first real chance with Charming. After a year (I like to imagine him sneaking longing peeks when he sees David around) he gets to meet his grandpa. And form a relationship with the first real male relative in his life (Regina killed two of his grandpas already, and neither mother has a serious guy. Regina never wanted Graham and Henry to have a relationship.) And damn, but he jumps in with both feet. Any time he can fit in a ‘Gramps’ it’s there. Because he has a gramps. And he’s an eleven year old boy who is excited to finally be able to acknowledge this relationship.