Play with him, brush him, massage him...touch and training are the two biggest things. It's easier if he lives with you though...if his foster mom can get on board with this, it will go MUCH smoother for everyone!
You said he's a ball fiend, use that to your advantage! Keep his attention on the ball when you walk and find that position to hold it in that will have him walking on a loose lead...don't worry about a textbook heel now, just not hauling on the leash all the time. But when he does hit that "sweet spot" use either a clicker or a verbal marker like "yes!" and then let him have the ball. You can either to a short toss so he can catch it, or if you're in an area where it's safe to do so, throw it and drop the leash so he can go after it (or use a long line).
Many dogs won't respond well to training until after they've settled in and formed some sort of a bond with at least one person. And if this dog has been bounced around, which is sounds like he has been, it's going to make it that much more difficult. The dog has gotten used to listening to himself and probably has some idea that it doesn't matter what he does or does not do, he's not staying in any particular place. I had a foster lab a lot like that and it was a PITA to work with her until she realized that she wasn't going anywhere anytime soon and then she began to open up and work with me.