Tips For Teaching Roll Over?

My dog is 7 and veeeeery mellow. Not sure what combination of mutt he is, but he's 110lbs and has a very deep and narrow chest. He sits, stays, walks well, and does just about everything else you can think of, but he just can't seem to figure out roll over.

I usually use food as a motivator, but he generally just lays down and stays there. He might swivel his head to watch the food. I've tried flipping him over to see if he'll get the idea, but he's too big to really move when he's in dead weight mode.

Could his bone structure just make him naturally not want to roll over? How do you motivate an unmotivated (but happy!) dog to do something?