Like superstorm said, kids being kids.
And we also saw that he only let his mom and Yuuto carry him, even when he was just a baby. So he always liked his uncle a lot and is jealous that the uncle he loves so much, gets along more with his dad than with him, even though both look the same (Natsu and Kai.) So it's just a kid wanting attention.
Why he never warmed up to his own dad, though, that's a good question.
I'm happy they are all happy and that Class Pres won. They even have a child now! I'd love to see more of the wholesome family stories of them all (without the grandparents, though.)
