Hey there,
As the previous message reccomended, get as many people as you can to help you with the search, the more the better. Also, look in places you wouldnt expect to find one. I lost my big male veiled, and I found him 2 days later, sleeping under a bush. How big is your chameleon??? If its a grown adult male, it may be easier to find at night, if you have no luck today. Shine a flashlight around, and look for his "sleeping colors" or the colors he pocceses when he's excited or sleeping.
I think I may be speaking for this entire forum when I say that all of us CANNOT stress enough to keep a close watch on your chameleon when outside, especially when they have no boundary's and are not caged. There are multiple threats to a chameleons just sunning himself on a tree, like birds of prey, cats, and any other animal. I experienced the same mess, and I was lucky that I found mine. I really do wish you the best of luck, and I hope you find him! 
Michele