Speaking with experience, you should never shave down a dog that does not have a continuously growing coat. It (obviously) will damage the coat and totally screw it up. You will end up with more undercoat then top coat and it will start looking fuzzy in areas that never had fuzz and mat up. Try to use a force grooming dryer to blow out his coat while he is dry. This will blow any loose/dead/debris out of the coat and you should be left with the nice coat (or what is left). Usually no one owns these dryers since they range from $100-$500, so if you have a local pet food shop they may have a self service pet wash and I guarantee they will have the dryers for you to use. Just take your GSD and blow him out really good all over the body and then give him a nice bath for his skin. There is also a coat supplement made by Davis that you just add to the food and it does wonders on thickening the coat in bald/thin areas. Also, LOL, if he is getting really thin you may want to think about putting sunscreen on any exposed bald area. the sun may burn his skin when he is outside, kinda like us...or me. Hope any of this helps. If anything else, just keep comfortable at his age :).