By continuously changing his food and trying something tastier, you are actually teaching him to become faddy. He'll turn his nose up at perfectly good food just to try and get attention or get you to try and ply him with something tastier. I suspect he's also doing it for attention. He gets given a new food and he eats it and then, satisfied that he's eating, you leave him alone rather than fussing about trying to get him to eat it. After a few days, he starts turning his nose up again in order to get that attention from you again because eating to food means he gets ignored. The solution...to stop rewarding him for not eating. Ignore him and just take the food up if he hasn't eaten it within a certain time.

Stick with the same food, feed him away from the other dog maybe and don't offer him anything else. No dog is going to starve itself to death and he'll get so hungry, he'll start eating whatever is available. Dogs don't taste as well as we do and it is in their nature to take advantage of any food source they come across. It isn't natural to be faddy, thats a learned behaviour that they get from their humans.
Don't leave his food down either. Give him 10 mins max in order to eat it and, if he doesn't, take it up and make him wait until his next meal.
Offering 2 or 3 smaller meals a day might help rather than just once....depending on how often he's fed in a day anyway.