Have you tried treats? When he brings the ball back, just sit there and hold a treat out until he drops it. Then as soon as he does, pass him the treat and pick the ball up. He'll soon suss it out.
And if you don't want to feed someone else's dog, just apply the same principle with the other ball. Hold it out as if to throw it, but don't do anything until he drops it. It may take 5 minutes, but after 2 or 3 goes where he gets it, it'll start to get easier as he actually understands that to get the other ball he must drop that one.
If you end up standing there for 20 min and he still hasn't dropped the ball, no biggie, it's game over. Try again next time. Like any game, if the rules are one sided, it's no fun. Except here, it's just harder to explain the rules to the other player :)
Should he wander off in boredom, just entice him with the ball if you're still up for playing. It's a matter of persistence I guess, like most things in dog training.
And to add a bit more, the very first time he drops it, make a MASSIVE fuss, make the game super exciting and the best throw ever. Make sure to associate a word with it from the start too, as soon as he drops it clearly say DROP, or something, but remember to make sure it's AS he drops it, not before. For now.