my son has the same problem. He's almost 9 now, and has a few adult
teeth that are discolored.
If I remember correctly, it also could be due to high fevers as an
infant, or other antibiotics, too. My dentist has recommended a
nightly Flouride rinse.

I only remember one high fever my son had (at < 1 yr old) as a
reaction to soy formula. Otherwise, he had the usual ear infections
until we figured out milk was that culprit!