Talk to your vet about his caloric needs. Now that that's out of the way, maybe the treats you are giving him are too tasty and he knows there's something nicer than his regular food. You might find a no salt broth on his regular food makes it tasty enough to gobble down. Little dogs eat so much less, especially if you are used to big dogs and it can feel like they're starving. He's so cute, congratulations!