That's very strange for A Chihuahua usually their very energetic dogs. It worry's me since Chihuahuas are normally very healthy breed of dog. I'm a Chihuahua owner myself so I did some research when I got my girl Cleopatra. Tho Chihuahuas get a medical condition called Hypoglycemia in which if the miss a meal they get very lethargic they need to learn to eat tiny meals through out the day. It's very common for Chihuahuas they have such tiny little body's and metabolisms they are you feeding the puppy a very high fat diet. Puppy kibble should be very fatty you want them eating all the time and the high fat foods even if they get a little pudgy it's all good because at about 10 months old you switch them to the adolescent formula and they lose all the baby weight. Either way you should see a vet shots are very important and the puppy should be fixed soon. Let us know how he is.
Welcome...You don't say if he has had his vaccines or not.....Either way, my first thought could be "Parvo" which can be fatal.....Hopefully not, but you need to seek treatment for him ASAP......Dehydration alone from not eating or drinking can be very serious in such a tiny guy.....You need to call your Vet for an appointment, tomorrow.....Do not wait any longer! I hope everything turns out for him...Good luck & please keep us posted....Karla