I wouldn't try to train it at this age. You're risking making fetch a job and not fun if you don't do it correctly and then your dog will never find it fun. He's still young. Many puppies don't understand the game yet. Most people will tell your their puppies didn't fetch until after 6 months or later. My dog would chase a ball, but he wouldn't bring it back, he didn't connect the fact that I'll throw it again, and again, and again.
I tried a lot of different things...cutting the ball and putting a treat in it, showing that a treat comes from returning to me, two ball, ect. The one thing that worked, was letting him mature and realize what the game is. When he finally started bringing the ball back, I'd always make sure to throw it again, and soon enough he knew what fetch was. The ball drive is developed at different stages, if you know the lines of your dog, and think that ball drive is something he'll have, I wouldn't worry about it and just let it develop naturally.