At least from what I'm seeing, its more of a "shading issue" if you want to call it that. Its more of, when shading or "Ambient Occlusion" was applied to the model, the backpack was on so the shading was applied with the backpack always being on in mind. It looks like this was added after the fact, so the back of him is just black. Only way to go about fixing this, would be remaking the texture from scratch, or a lot, and I mean a lot of good photoshop experience.