Bless you, for giving this guy a good home. I have a hard time not bringing them all home with me, but I know that would not be good for anyone!
We played the "Name Game" with Parker when we first got him. He's very independent as well, but this works pretty well. It is treat intensive, but after a certain amount of time, they just associate coming to you when called with good things happening. It takes two people. Stand a distance apart, each with several treats. Take turns calling "Charlie!" just one time, and when he reaches you, he immediately gets a treat. Then, right away, before he can turn around or lose interest, your partner calls his name and gives him a treat. Then you will immediately call him back.
When he starts to run automatically back and forth between you, change it up, so that he understands that the game is to listen for his name, rather than to run from person to person for treats.
Parker is also a shelter dog, and his quirk is that if I have food, and I'm maybe watching tv or something away from the table, he makes this HORRIBLE, gut wrenching growl if any of the cats even looks like they're going to come near me. I don't know if he thinks he's defending me from them, or defending his own spoils (which he never gets!) from the cats. It's very strange, but he's never hurt anyone and I don't think that's his intention. When there is no food involved, he's very affectionate with the cats and takes good care of them, snuggling when they need and gives them lots of kisses.
I hope that helps. :-)