Puppies grow quickly and have special nutritional needs to keep their bodies strong. The most noticeable time is known as the rapid growth stage, which occurs between 2 and 6 months of age. From about 3 to 6 months, most pups will lose their puppy teeth (you might find a tooth near his dish or on the floor; this is normal).
During this phase, his mouth and gums may be very sensitive, which could cause him to be a bit picky when it comes to eating. There's nothing you can do except to wait it out, even if his appetite is low for a couple of days.