First thing you need to do is put that dam-n gun away!
When I can see hogs that far away and the dogs haven't seen them I like to get up on the back of the truck and hold the dog's head, pointed in the direction of the hog... all the while talking to him, pumping him up... while all the while driving closer to the hog...
Doesn't take many times of that to get one looking "long range" for hogs. Once they figure out the idea, I think dogs actually have better long range vision than people.