Try having him "lay down" and hold a favorite toy or treat in front of his nose. Then slowly raise your hand with the toy/treat back over one ear and towards his tail. Basically getting him to follow you with his head.

So now your dog should be in a "down" with his head looking back and toward the sky. Now just continue to move your hand with the treat back and crossing over his back.

The only way he will be able to keep his eye on the treat will be to roll over. After he rolls over, give him the treat and a ton of praise.

Rinse and repeat using your voice command of choice.