Not familiar with the scene leading up to this, but it appears that Andy is cleaning his pistol. As such I'm sure it's unloaded.

While Trey is to Andy's left and the gun is pointed to Andy's left, I don't think the gun is pointed right at Trey. It appears to be pointed well paste Trey's left shoulder, or there abouts.... as I see it.

Better to show the boy how to properly handle a weapon at this age, than to have him pick one up, with no supervision a couple years later.