There are many ways to teach leave it. In my class we teach leave it by dropping a piece of food on the floor behind us. When the dog goes for the piece of food, we body block the dog from getting the piece of dropped food, pairing that with a 'leave it' command. When the dog stops moving toward the piece of food on the floor, we give them a treat with a GOOD BOY/GIRL! Once the dog understands that, put the dog on a lead with pieces of food on the floor, just out of his reach. Walk him past the food on the floor, as he starts eyeing the food, tell him leave it, when there is the slightest inkling that he is going to listen to the 'leave it' (even if it's just a turn of his shoulder toward you), pop a great treat in his mouth and praise while you are walking past the food on the floor. Practice, practice, practice. If you are going for CGC/TDI certification, this method works well as they do leave food and toys on the floor for testing and it translates to other things as well, dogs, squirrels, etc.
If you want your pup to 'give' the ball, I would do as Jackie suggested to trade for another ball OR the yummiest, smelliest treat you can find. It sounds like Mason might not be too food motivated or you aren't using the tastiest treats possible? The treat can make all the difference in the world to food motivated dogs. A slightly hungry dog is more motivated to train with treats than one that has a belly full of food.
Edit: I should add that the dog/puppy NEVER gets the treats on the floor.