I am puppy sitting and the dog won't eat. Suggestions?

I am watching a one year old retriever. I have him for almost a month, so he needs to eat. He is supposed to be getting 1.5 cups twice a day. he has only eaten his meal twice, so all of 3 cups, in the 6 days that I have had him. The first day when he didn't eat I wasn't worried, I just figured that it was because he was in a new environment. When it went on four days without him eating (and I had already tried giving it dry, mixing warm water in, mixing cool water in), I went to the store and got a bottle of that Iams gravy. He is on Iams lamb and rice. I wish I could change him to a better food, but he is not my dog, so it is not my decision. After mixing in the gravy with the food, he ate the whole meal, and the next morning he did the same, but now he is not eating again, even with the gravy. I even tried warming it up. He has been pooping a LOT, which I am uming is from the low quality of the food, as I am used to my dog going a small quantity once a day, and he is about the same size as the dog I am watching. Last night he threw up at about 4 am, it was mostly stomach bile with a bit of hair. I have him for another 3 weeks, and I need him to eat! Any suggestions would be appreciated.