Your little guy is absolutely adorable. But remember, there really is no such thing as a "teacup" chihuahua. (It's just a term some breeders use but it's a misnomer.) Some breeders just systematically breed small males to small females (often the runts) to produce tinier and tineir dogs. Unfortuantely, these tiny cuties often come with a host of problems their larger breed counterparts do not. I'm hoping your little guy is healthy as a horse, but when we breed just for tiny, we often get unhealthy animals.
Please understand, I'm not trying to be negative, just informative. I'm happy this guy has found such a wonderful home and you already seem to love him. But as a vet tech, I'm just speaking from experience. So enjoy your little guy but be aware that he might have some issues you'll need to deal with. As with all dogs, get a good vet to give him a thorough exam and help him stay as healthy as possible.