I understand veileds are a bit more aggressive. He may be acting like a healthy chameleon. The common chameleons I used to wild catch in Libya were very aggressive and I have to believe that the Yemen Chameleons are similar. Chameleons in the wild also have very large territories so I imagine even the nicest cages are small for them. Make sure he has plenty of climbing places and I wouldn't worry about the aggression. Senior members can let us know if increased handling will help with the aggression.