Did he see you? Some chams drop to the floor when startled. Pan did it the other day when he saw my hand in his cage all of a sudden (I thought he knew it was there) and he fell into his pothos vines and down to the floor a few inches from the last of his vines. I felt kind of bad since it probably could have been prevented, but he was fine.
Chams are built to fall to the Forest floor in the event that they need to flee quickly. A PVC bottom isn't as soft as leaf litter, but they also aren't always falling from as high up. Just check him over for injuries and keep an eye on him for a while.