Hi,
Half the answer is in your question "He is very energetic"..... in other words he spends a lot of enery (the result: he is thin). Don't worry about his weight, he will surely gain weight while he grows. As per the prefered food: The best think to do is to try all kind of brands (for dog's food). Soon you'll find out which one your dog prefers. Please don't be hard at him, he is a puppy still...... meaning he is a baby and is still trying to learn all what the world is offering him..... better teach him what to do and what not to do and you'll get your best friend doing exactly what you like. It's a GR8 dog and very strong, never underestimate his powers.
Enjoy the weekend,