|First, stop giving him people food. I know it's tempting to do whatever it takes to get him to eat, but don't do it. People food isn't nutritionally balanced, and chihuahuas have such tiny tummies that they really can't afford to eat anything that isn't packed with the right nutrients.
Has your vet told you that he is underweight? I assume he is under veterinary care, since he has been wormed several times. I do wonder why the vet is deworming him unless puppy has worms in his poops. If you are doing this yourself, Stop It! De-worming is strenuous and shouldn't be one unless there is a demonstrated need. Your puppy should be under regular veterinary care, if this vet isn't familiar with chihuahuas (or toy breeds in general)than you need to find one that is.
The reason I ask what your vets opinion on puppy's weight is that I have two chis. One is built like a supermodel, thin and dainty (3 pounds, size 2 in human terms), a lot of people think she is underweight because you can see her hip bones. My other is built much more sturdily (5 pounds, size 8 or 10 in human terms), next to her sister she looks fat. Both of them are just fine, have energy to burn, and according to my vet are perfectly healthy.
As to why he's not eating dog food, why should he when he can get you to feed him much more tasty people food by being picky? He's trained you very well. If he has plenty of energy, he's not starving. Maybe he liked the Nutra Nuggets because they had a lot of sugar or etc. that made them especially tasty. I'm not familiar with that brand. Searches of the web found that other brands recommended just as much are Waltham Lamb and Rice and Science Diet (which is what I usually feed mine, although I have been known to mix in Royal Canine, Natures Choice or Purina one when they have Free-after-rebate deals).
You may be unaware of how little a chi really eats. I found that even now, it is unusual for either of mine to eat more than 1/4 cup per day, spread out over two meals. Even with the difference in size, they both eat about the same, as little Gremlin is more active than her larger and mellower sister Nixie. I subscribe to the belief that food should always be available, the dog will eat when it is hungry and stop when it is full. I see that with both my girls, when they're full they don't want any more, even if Mommy tries to coax them. They did prefer smaller kibble, I started them on Science Diet small-bites puppy food. Now that they are adults, they will eat the larger kibbles as long as they are flat rather than chunky. Also, my larger chi doesn't like to eat or do her business in front of me, she's very shy. Perhaps your puppy is sneaking food (or being snuck treats?) when you're not looking.
I strongly suggest you use a vet that gives you a feeling of confidence and can answer your questions, ask for the names of other clients who are happy with the care their small breeds receive. An uncompensated plug, I use Banfield Pet Hospitals which are all over the country. Also, please have him neutered. Only excellent examples of breed standard, e. g. champion show dogs, in the care of experienced breeders should remain intact. He won't miss it, and will be much happier and less likely to scent-mark your house, as well as less likely to stray in search of females.
A very good book is Chihuahuas for Dummies, the best I have read on the care and nurturing of these adorable creatures.