As someone who owns Newfoundlands and has worked as a groomer for all breeds - DO NOT shave the dog!! No double coated breed should be shaved. I've heard the comments about the "change in behavior" for years. The problems are: 1)Yes the dog can get sunburned. 2)The dog has far less protection from thorns, sticks, and teeth of predators that he is defending your flocks and herds from. 3)And when you shave him, you cut off all the outer coat, which is what has the water repellency - 4)this means that any time he gets wet, he gets wet to the skin. 4a)This has its own set of problems - he can get chilled quickly(have you ever gotten soaked while wearing jeans and a cotton shirt?) and 4b)you set him up for all kinds of skin troubles.5)As many people have already said, the coat keeps the dog cool -IF it's properly cared for. Keep the dog groomed out so dead coat doesn't block air circulation, and the outer coat acts like old-growth forest. And what happens to the ground when all the trees are cut down? Right - it gets HOT! So if you shave your dog's "trees", his body will be hotter because the sun can get to it easier.
IN the end, it's your dog, but my experience would lead me to shout, NO, NO, do not shave your double coated breed!! Keep him brushed out instead. Have your groomer show you how to be sure you are grooming him right down to the skin, not just the top layer. When my Newfs are groomed every week or two, it takes about twenty minutes to do a thorough job. Don't ignore tails, leg "feathers", underbellies, armpits, and behind the ears, they become nightmare areas if left ungroomed.
Best of luck, and enjoy your dog's beauty in the way it was meant to be!