All puppies have to switch food eventually. Whether yours is ready for adult food or you just want to switch to a new brand, you have to switch over gradually. Some pups need a little extra motivation, so you have to make him want his food instead of forcing it.
Introduce new food gradually. If you expect your puppy to switch completely from one food to the next without any transition, he may reject the new chow. Over the course of a week or so, gradually mix the new food in with the old food. Decrease the amount of old food every day, replacing it more and more with new food -- at the end of the week, his diet should consist of only the new food.
Work up an appetite in your puppy by exercising him before mealtime. A lethargic puppy doesn't need much in the way of calories, but one that gets a good daily walk and plenty of playtime has a healthy appetite once the food rolls around.
Stop doling out the treats. If your puppy is getting nutrition or even just calories from treats, it makes it easier for him to ignore his food. While you can still give him treats now and then, he has to earn them by obeying commands -- giving them out for no reason isn't a good habit for either of you.
Experiment to see what type of food he responds to. Some dogs are just finicky eaters, and you have to determine what he likes. For example, if he rejects a new dry food, try feeding him a new canned food. While canned foods may be best in moderation for the adult dog, as a puppy, the extra calories won't hurt.
Take your puppy to the vet for a checkup. If he's not eating at all, the problem could be medical, and he needs to be checked before it gets any worse. Puppies in general need to eat a lot so that they grow properly, and ignoring the problem could have severe consequences for the little guy's health.