My guess is that the kibbles are too hard for such a small puppy. Try canned, dampening the kibble with some warm water, or smaller a smaller kibble (or a combination of the three).
That said, this puppy is going to stay pretty small. Some dogs, with good reason, just do not like commercial dry diets. You could try a raw diet, either commercial or home made. I’m currently using See Spot Live Longer premix (which is off the market for a few months right now while they set up to be able to produce a larger quantity and other formulas) and various meats for my 8 lb Jack russell mix, at about $20 a month. There are other premixes as well, and they’re pretty easy to find, with a google search. If a raw diet isn’t something you want to try, there are also premixes for home cooked diets.
