Hitsugaya-now is trying to live up to his standards of a Captain, in the body of NOT a kid, of a young adult-who happens to be very short. He's a child prodigy, but he's definitely not a kid . And on top of that, he doesn't act anything
I don't believe he acts any different around Matsumoto then he would any Captain. He's still arrogant, rude, and tempermental; he still demands respect despite his height, and won't hesistate to call you can idiot . On top of that, he's still "human"-he sleeps and eats, and stuff xD. He's not one of those guys who runs anger 24/7, so its not like he never has fun or anything, he just puts his obligations before his personal time.
Honestly, Hitsu-kun is all for showing that he's Hitsugaya Toushirou-Captain of the 10th Division first, and a young still growing soul underneath all that junk :d