Welcome to the world of gsds! They can be the pickiest of eaters and you'll be a poop expert before long. You might try brown rice because it has more minerals than white and either cooked hamburger or chicken, then add a 1/2 c of his dry to each meal and gradually increase the dry and reduce the rice/meat mixture. You might also need to switch to a grain free or chicken free food- Stosh can't tolerate chicken, which isn't all that uncommon. You might have to try one or two brands of food before you hit on one that he likes and doesn't upset his stomach, but you also don't want to fall into the trap of bribing him to eat. You didn't say how old he is, but I imagine he's eating 2-3 meals a day, so once his stomach has settled down, put down his meal and give him about 15 mins to eat it and if he doesn't, pick it up and don't give it back until the next meal. It won't take long before he figures out that he has to eat when it's offered or it's going to disappear.