If he lost interest when you upped the amount of food, go back to the amount he was getting when he was scarfing it down. It may be that he's just being fed too much and he isn't hungry enough to eat it all. 9 cups seems like A LOT to me.
Also, I would not add anything to his food to get him to eat. That will just lead to a picky eater. Put his food down, and if he doesn't eat after 10 minutes, pick it back up and don't give it back until the next meal time. He won't starve himself. Eventually he'll be so hungry he will eat it plain.
Others will likely mention that that isn't the highest quality food. You may want to make a switch.
Forrest - GSD 9/1/2016 - 5/14/2017 RIP
Brooklyn - Golden retriever 1/30/11