At first aim, he can't shoot it cause the old man told him not to do (cause it's grass eater or something).
But in 2nd aim (on page 12-183) I think translation is wrong cause he only said "It's" and "Ummm..." but I also have the original manga (local version that translated and sold in my country), in original manga, he said something like "well you listen to me, that dinosaur is......" and BOOM head explode.
IMO he tried to explain to the old man that that dinosaur is NOT harmless but he accidently press the trigger when he point the gun to it.
Ps. Sorry for my english