There are a number of things you can try.
1. Always approach slowly. Make sure he sees you before you are actually at the cage. Hesitate while still a distance from him and allow him to process the fact that someone is approaching, and give him a chance to recognise you from a distance. Move your body and your hands and arms slowly.
2. Always wear cham-friendly colors. A chameleon expresses his mood and his intentions by coloration and he assumes you do, too. Wear pale greens, khaki, bland browns. I keep some oversized shirts handy that I can throw over my regular clothes when necessary. Avoid bold patterns and bright colors, especially pinks and reds. And avoid black. Black may appear menacing to him. Throw a khaki shirt over your shirt if it's not a good color. Only takes a second and makes a world of difference.
3. Respect his space. When he lunges, draw back. If he gapes and carries on, walk away. In doing so you are showing him that he has some control over his environment. THis is very important for a male chameleon. You have some control, but he needs to know that he has some control, too. This will boost his self esteem and confidence and give you a healthier, more well-balanced chameleon. Initially, with a male like this, I place his food in his cage in the least invasive way and close the door and walk away immediately. I let him watch me putter around the room day after day. Soon he will stop viewing me as an immediate threat. Then I will be able to approach him a little more, offer treats on a vine not far from him, and eventually try hand feeding.
We've taken in rescues, wherein I have had to make friends with an adult chameleon who has not been treated well. With these methods I've been able to establish a rapport with some of the more difficult chams in just a few weeks.
