It's possible that the guy with the guy (mistakenly?) believed that he couldn't legally wear a mask while carrying a gun, like the person in Crazy Medic's post. If that's the case, then his actions seem reasonable. He first informs the worker that he is not allowed to wear a mask / has an exemption, but does not reveal his gun. Then the worker specifically asks the man to "show it to me" and he presumably pulls up his shirt / jacket to reveal his gun.
I have no idea if Kansas has a law on the books about not wearing a mask while you are carrying a gun, but I could see how such a law would make sense (in normal times). Like some state's laws on not wearing a mask while engaging in a protest, etc.
The kid's statement of "I ask him to wear a mask and now he's going to shoot me", is either complete overreaction, or an extreme case of gun phobia. This of course assumes that the guy just showed him the gun, as the kid asked, and didn't draw it, etc.