Nelson knows that a stomach wound is fatal, and that the ambulance drivers, if they even came, would not take him to be treated by the surgeons. Nelson faces "an agonizingly slow death - it might take two days - and ...pain (that) left men screaming until they were too hoarse to make another sound". Rather than die like that, Nelson plans to take his own life. He asks Charley to reload his rifle "just in case the Rebs come back", and to remove his right shoe because of "a powerful itch" on his foot, leaving the fact that there is no hope for him unsaid. Nelson plans to shoot himself with his own weapon, pointing the muzzle towards himself and pulling the trigger with his toe. Charley, though he tries to feign nonchalance, knows that there is nothing he can do for his friend, and so complies with his last requests (Chapter 6).
Well, Nelson knew that he would die, because stomach wounds were fatal and doctors back then couldn't fix stomach wounds. So, Nelson would have had to go with an agonizingly excrusiating slow death, but instead he didnt want to go through the pain and wanted to die quickly so he asked Charley to load the gun for him and he shot himself.