At 12 weeks, I’m not sure many puppies are truly picky. There are a few, but i think it’s less common in retrospect. If I had to guess, it’s probably a combination of things including adjusting to his new home, possibly not being food motivated (yet), still learning the mealtime routine, testing limits as to how far he can push before getting something better
The most important thing is that you are the one who chooses what your puppy eats, not your puppy. It’s not mean, I promise. Whatever you give, he will most likely learn to enjoy it as long as he doesn’t get something “better” when he doesn’t eat. Evaluate the serving size with your vet if you need help (like Ricky’s Popi said, the serving sizes on packages are all wrong, and servings should be determined by calories, weight, and monitoring growth) and pick up the bowl when the meal is over, don’t leave it out. Overfeeding is really common, especially if you use the serving sizes on the package, and more likely to make him not hungry.
There are lots of threads on different options for food, to fit your nutrition philosophies, budget, and lifestyle, just remember it’s up to you
Don’t give in to his cute puppy face, because he’d probably be perfectly happy eating pepperoni pizza for every meal and we know that’s terrible for him!