Well when I say specific, I mean after he has eaten a certain type of food for a couple weeks, he will no longer eat that specific food. Put a new type of food in front of him and he will eat it. So he isn't just off food 100%, he just refuses to eat the same thing for more than a couple weeks.

He occasionally does have loose stools but it doesn't really coordinate with when he doesn't want to eat. He's an outside dog so I figure he will occasionally eat something that will give him the runs but it is never persistent...

He's probably just a clever dog who gets bored with the same food day in, day out.

I think he is a smart cookie, who has learned that if he refuses to eat his food he gets another choice. I would buy one good quality kibble, put some kind of wet food through it and put it down for 15-20 minutes. If he doesn't eat it, pick it up and give him nothing else, then put the food down again at his next meal time. If he doesn't eat it, same thing again. No healthy dog will willingly starve himself.

Have you tried feeding him a raw diet? Or giving him a raw meaty bone? Eating bones creates saliva and the enzymes in the saliva is necessary to help build their immune system.

Dogs need variety, and kibble is hard to break down for a dog anyway. If I were you I'd switch to raw.

In my opinion, this is a sign of a GI imbalance. Research came out a couple of years ago that most puppies and kittens are now born with defective gut. It seems that over the years, from feeding inappropriate diets, there has been some genetic mutations that have occurred in the cells of the gut. Unfortunately, these genetic mutations are passed on to the offspring. Recent research has shown the same thing happening with people and their children. That said, it is not uncommon to see early signs of digestive problems, such as anorexia, soft stools and/or vomiting. My first suggestion is trying to do a slow transition to a balanced, fresh diet, preferably fed raw. I would also recommend that all puppies are started on a digestive enzyme that would help the gut function. There are many available online. The one I recommend (if I am allowed and I am certainly not affiliated with) is Dr. Mercola's digestive enzymes for pets. Since your dog is still a puppy, these changes should correct the problem. Good luck.