Soldier sounds like a fearful dog. If you try to use Cesar Millan's method on a fearful dog, you could compound the problems he has. I'm not a dog trainer, but, I have experience with dogs who were abused. For me, what works best is to gain their trust. The easiest way to do this is through basic training.
With my dogs, I spend about 5-10 minutes every evening doing simple training: sit, down, shake, stay, come, etc. The first week or two, give him a small training treat after every command that he obeys.
After awhile, you can start to reduce the frequency of treats, but, still use those treats. I would suggest doing this when immediately after your girlfriend leaves for work. It will create a better bond between you and Soldier if she is not present. But, let her know what you are doing and why you are doing.
If Soldier is initially reluctant to do training with you after she leaves, start out feeding him the training treats for a couple days without requiring any action from him. Give him one every minute or two, and limit the treats to no more than 10. When he starts expecting the treats, then you can start the training. Be patient, don't get upset if he doesn't immediately respond to commands. As Soldier is not a puppy, it may take him time to get the hang of the various commands. Around January 1st I adopted two pit mixes (they are both about 5 years old). It took me about 4-5 weeks to get one of the two dogs to do "down." But, now, he gets the command and does it eagerly to get that treat.
Don't force him to do anything (like pushing his butt down to make him sit). Use the treats to get the right behavior. The page linked to below has videos and good tips on how to do that:
I had a chihuahua that had clearly been abused. He would squeal in pain if I tried to scratch him behind the ears, and, he reacted to a belt in my hand (I was getting dressed) similar to how Soldier reacted to the leash in your hand. He eventually came to trust me, and looked forward to me scratching him behind the ears. It took a while, but, I also did make it clear when he something wrong. I would shut him up in my spare bathroom for 1/2 hour to an hour. Usually once or twice and the unwanted behavior stopped. I didn't need to hit him, or physically intimidate him to get the message across.
You and your girlfriend need to get on the same page as to what behavior is allowed, and, agree on punishments when he disobeys. Putting him in his crate, and covering it with a towel or blanket may work as a punishment. It sounds like he craves human companionship and depriving him of that when he misbehaves will go a long way toward curing him of bad behavior.
I've used positive reinforcement methods to train all of my dogs (all the dogs I've had as an adult have been rescues who were at least 3 years old when I got them). The first dog I had, I didn't start doing the daily training until I'd had her for about 6 months. But, after several months of doing the daily training, I noticed she responded to my commands much more quickly when we were away from the house.