My baby veiled chameleon was hanging off the ceiling of the cage and his mouth was open he just finished eating and drinking. Is it normal for them to chill with there mouth open?...he closed it when i touched him.
He was probably too hot. Opening the mouth is a way they cool themselves.
If he is getting on the ceiling of his enclosure right under a lamp, you may have to raise the lamp. Baby veileds do not tolerate the basking temperatures that adults of the same species enjoy and they do not thermoregulate well at that age.
While it is most probably overheating like Brad said, open mouthed breathing can also be a sign of an upper respiratory infection. If he continues to do that, or if he shows other signs of being ill, be sure to take him to the vet. The earlier you catch it, the better chance that medication will be able to help your little guy out.