Welcome Brandi and Lexie!
Lexie, I would be quite cautious about buying a quiet, docile puppy. Usually puppies that can be described with those words are not healthy, unfortunately. An adult like that is one thing, but puppies should be bouncy and bratty. The fact that he is half the size of his brothers would also be a red flag for me. I have met healthy puppies that size, but they were little spitfires. I would encourage you to ask the breeder if you or she can take the pup to a vet of your choosing for a good workup (blood, stool sample, etc.). Things like liver shunts and hydrocephalus are relatively common in these tiny guys, though it could just be worms or another parasite. If it is a parasite infection, don't expect him to stay so mellow! I speak from experience!
First time Chihuahua owner with many years of dog experience!