Does he ever lay on his back of his own free will? If he does, then as a first step you can reward him with a treat whenever you see him doing it. Then you need to gradually work toward getting him to do it when you give the signal.
If he doesn't do it on his own, you'll have to work toward getting him to let you roll him over as the first step, which will be harder to do. At first you would reward him for letting you tilt him just a little bit, then gradually work on tilting him further and further before he gets the reward.
Positive reinforcement training aka clicker training is the way to teach birds tricks like this. It's more complicated than it looks, and it doesn't come naturally to me. I have a very basic article about it here:
http://www.littlefeatheredbuddies.com/i ... icker.html but you'll get a lot more guidance from other resources. My information on this is pretty old and I'm not well informed on where the best groups for this are currently. But if you look around you'll probably find an active bird training group.