Ben acted like the slash from Pennywise with the spear was a nothing
So Henry Bowers carves an H into his belly and he acts as if he's dying, they do a whole big thing of them rushing to the pharmacy to get him bandages and medicine.
But then when they're in the creepy house and after Beverly sticks the spear into Pennywise's face, Pennywise slashes Ben pretty badly in the stomach and yet he acts fine. They're all standing outside the house and Ben is just standing there not even holding the gash on his stomach, its as if the actor forgot he was supposed to be feeling pain and that he just got slashed in the stomach.
Richie even points it out when he says "And Ben is bleeding hamburger helper over here" and Ben continues to just stand there with that big slash across his stomach as if its nothing.