The response of Eckels when he sees the tyrannosaurus rex is to completely lose faith that it can ever be killed, and he looks at the gun in his hand and sees something that will never be able to even hurt the massive and formidable creature in front of him. Note how this is expressed in the text:
"It can't be killed," Eckels pronounced this verdict quietly, as if there could be no argument. He had weighed the evidence and this was his considered opinion. The rifle in his hands seemed a cap gun. "We were fools to come. This is impossible."
He then is told to make his way back to the time capsule, but he is so shocked and dazed by the presence of the dinosaur that he staggers off in the wrong direction and falls from the path that is meant to keep the travellers from tainting the past. As the others shoot the dinosaur, he is left there, until he finally makes his way back on the path. He is then made to dig the bullets out of the dinosaur before they all leave.