Don't leave food down. Pick up the bowl after 15 minutes if he isn't eating and back down at the next meal time. You might also reduce the feed to what he actually eats so he gets used to seeing an empty bowl. If he gets skinny increase that amount a bit. Suspect he is already plenty skinny being an adolescent GSP and all!
Dog food bags usually suggest more food than most dogs need and/or your pup may be past his major growth and need less food now.
IAMS is pretty eh stuff, he may not like the taste much. If he is so skinny you can count his ribs and vertebra without putting your hands on him he may need a different food or you could experiment with giving smaller than usual amounts [so less is wasted if he doesn't go for it] mixed with warm water, broth, a bit of canned food, a bit of fresh egg, canned fish or other meaty stuff to see if he eats it more readily. If he seems to like the idea increase the amount a little at a time. You want him to polish his bowl, no leftovers!