He's an American Bulldog btw.
The thing is, he won't give you the ball back. He will chase it, get it and come back but he will just stand there with it in his teeth. I tried food (he's normally VERY food oriented but it doesn't work at all), tried trowing other balls (the other ball even if it looks the same, doesn't interest him at all). He shakes his head and tries to get it out of my reach when I try to get the ball.
I tried giving corrections (won't budge), waiting doesn't work (he will gnaw at it for a looong time), and when I don't budge - say leave it for 15 mins and when he tries to lay down to gnaw at it I make him sit he will whine with the ball in his teeth.
The only thing that works is when I poke his tongue with my finger and even then I have to keep him upright by the collar or he will dive after the dropped ball. He's concentrated 100% on it (doesn't notice other dogs etc) and I don't know how to let him know it's okay, we will play if he gives it back. He's really hyper about it too so stopping the play in the middle and picking it up later doesn't work (he's too zoned in on the ball to remember why we stopped in the first place).