Sometimes Loki picks at his food and I need him to eat, like before training class (So he won't be a monster and attack the treat bag because he's hungry!). I will put a couple of tsp of chicken stock on his food. I only use the good stuff since that's what I cook with and have around - the low sodium stock not the campbells type broth. This makes him want to eat. :-) You might regret giving gravy since it might cause tummy problems. This isn't always the best idea though, because if he's not eating it there might be a reason.
Also, I just want to point out that none of those foods are really good foods. I know you said the vet gave you some but vets don't always have the best advice when it comes to foods. You need to read the ingredient list and find foods with 2-4 meats in the beginning of the ingredient list, no meat by-products, no artificial preservatives and no corn. Some suggestions are foods by Innova, California Natural, Canidae and Wellness. You can get these at Pet Supplies Plus, some feed stores and a lot of small independent pet stores. Just read the ingredients and you'll understand why he doesn't want to eat it
Good luck!