you need about a 30' piece of lightweight line. Think REALLY long leash. toss the ball. when he gets it call him, and pull him towards you. when he gets to you show hit a LITTLE treat. when he drops the ball give him the treat. then...... Repeat. repeat. repeat.
Start off not throwing it, just drop it. Then get him to bring it to you and praise him. Some dogs aren't naturals at this, so be patient. Make a fuss when he brings it back and drop it again. Eventually he'll get that it's a game.