I would go to the vet and have him run blood tests to see what his levels of everything are. He could have a severe imbalance of one thing or another, and I almost want to say calcium, even though he doesn't present with the physical deformities of severe calcium deficiency.
The reason being because calcium isn't just there to give you strong bones and white teeth, it's a mineral that plays a part in every single nerve signal your body sends to move any and all muscles so if you don't have calcium you don't cannot contract your muscles properly. So I would suggest that he runs a blood test unless he can successfully diagnose it as something else (neuro, injury, etc.) without doing so.
At any rate, no one is going to be able to help you as much as a vet that is experienced with chameleons.