If anything he was more immature in KH II than the first game.
This is what i couldn't believe. In KH1 he seemed somewhat more mature than his age (and size) would suggest, but in KH2 he seemed likea little kid comparatively. The worst example of this was the scenes during your first run through of Twilight Town. (Though I must admit that I LOLed when he shouted "And then we can go see Santa!!")
I also couldn't believe how silly this game was compared to the first. KH1 was more mysterious and interesting, but KH2 was more like an adventure in a cartoon. And all of the events on the worlds (Save TT, TWTNW, and Hollow Bastion) were completely isolated!!! No real point to add to the main story whatsoever.....