dominance exercise

I've found with large dogs it's hard for them to be on their backs.  I do the dominance exercise with both of my dogs, a pomeranian and a gsd.  With the pom it's easy to just flip him over, but i usually do it on a couch or bed so I dont cause him any discomfort.  With my gsd, I give the hand signal for down, then I gently push him so he's lying on his side.  He's fine with this.  And I just do the dominance exercise with him in this position, checking all his paws, ears, teeth, etc with him lying on his side.

You might find this easier than making your dog lie on his back, since it's physically uncomfortable for a large dog to do so.

Restraining for massage is NOT a dominance technique - it's just a way of insuring that if you must restrain your dog in a supine position for x-rays, etc., that he will not be freaked, and will accept gentle restraint while on his back.  We normally do it with puppies, though.  It really isn't something I advise with large adolescent or adult dogs. 

I would not worry about it on his back.  I would work on fully relaxed laying on his side.  If he will lay on both sides fully relaxed, you have complished about the same thing.  I have one dog who will not lay on his back under any situations.  He will rotate slighlty and lift his legs for pets, but not on his back.  He is also quite lean and has minimal tissue over his spine.  It may not be comfortable for him.

Our trainer has a GSD who will not lay on his back because his spine is "pointy". She recommends using this techinque to calm them and groom them, not necessarily as dominance training. Her lab will roll right on his back and even fall asleep in that position. She just has her GSD lay on his side between her legs and accomplishes the same thing.

I would suggest you being calm, gentle and insistent. Go slowly, speak praise softly. Release as soon as you get a hint of his relaxation (submission) and reward with a treat. I'd release him gently - and not make it exhuberant.
 
Try in low distracting places he knows well, in your home. Soon enough it will become a fact of life.
 
(hmmm, I have to work on this myself)

I trained my dog to lay down on his side with a treat lure and clicker. (For us this is "play dead" but also the position he gets into to be brushed.) He resisted at first, and understandably so as far as I was concerned... not knowing why I wanted him to get into this vulnerable position, he was hesitant to do it. But after many short, relaxed practice sessions, he got comfortable with it. Now he'll flop right onto his back in an attempt to weasel treats out of me. []
 
I started by commanding a "down." Then, kneeling beside him, I'd hold a treat close to his nose with one hand, moving it over his head and to the side, while keeping my other hand on his ribcage so he wouldn't try to stand up, and gently encouraging him (physically, just pushing softly with that hand) to roll. At first I would click & treat for any slight movement in the right direction, and finally he'd begin to roll to the side.
 
He still won't lie flat on his back, I don't think it's comfortable (on our hardwood floors), unless sometimes when I'm brushing him and he's relaxed enough I can roll him onto his back with his body wedged between my legs for support - but I have a small dog. []

It took us MONTHS to get Roscoe willing to accept being on his back and having his tummy rubbed.  He simply didn't trust us enough 'til then.  It took lots of desensitization. First we had to get him to accept having his tummy touched at all.  Then let us do it while he was sitting, then lying down, etc.  We used a very similar method to jones and Russell's; it just took us longer 'cause Roscoe was an adult dog who isn't as submissive in the first place. Even now I can tell he likes having his tummy rubbed and massaged, but he's kind of nervous about letting me do it. 
 
Oh, and he's actually not even on his back. He's on his side. He has never been an "on his back" kind of dog. I think it's uncomfortable, so I don't force it. As long as he relaxes and lets me work on him, that's all I ask.