My Labrador puppy is too thin. What should I feed for good health?

Labradors are large breed dogs and thus they should eat a large breed puppy food to help correct growth and development. The large breed puppy formula will make sure growth is slow and consistent which is best for development. Large breed dogs are prone to a variety of growth abnormalities and the appropriate diet helps to prevent them.
Additionally, most veterinarians do not recommend grain free diets. The research is not fully complete but there appears to be a link between grain free diets and heart disease. Not every pet who eats a grain free diet will develop heart disease but more dogs do on these diets than dogs who eat grains. There is likely a genetic predisposition that the diet affects.
I recommend to change your food to a commercial diet that is formulated for large breed puppies. I recommend the brand Royal Canin. They even have a formula for Labrador puppies. This diet is easy to digest and is formulated for appropriate growth. This diet is complete and balanced and you do not supplement protein or vitamins/minerals. It is also important to make sure that Pom is dewormed appropriately as parasites will cause dogs to be thin. He will gain the weight once he is on an appropriate diet and eating the correct amounts.
I hope this helps and please feel free to post any additional questions.