Oncebitten - I have never had a dog myself with guarding issues but I have seen a dog with it. We have always taught from a young age that food, toys etc you get when i say you get. One of the first things I religiously teach my dogs is to "leave it" where you hold food right near their nose and they don't touch it untill you say.
I would agree with the high value treat or making sure you have two of everything so that You can trade the ball you have for the ball he has.
Teach him to sit and stay, so that you can get him to now want the ball you have, move away from the original ball where he then sits and stays while you pick it up. Treat him with a snack or a throw of the ball.
Continue that till he realises its more fun to give you the ball back cause the game keeps going.