If your chameleon was a dog or cat, a vet would say he was experiencing neurologic symptoms consistent with a central nervous system deficit or lesion. If he can move all of his legs, then most likely the deficit is located in the brain. If the pet could not move his legs voluntarily, then the lesion would be more likely located down the spinal column, the farther down toward the tail indicated by which legs he had control of function.
The symptoms you described could be caused by anything from seizures, a temporary pinched nerve, inner ear infections, encephalomyelitis, parasites or a virus invading the CNS, trauma to the spinal column, a blood clot or bone fragment (or other foriegn body) wedging in the brain vessels or spinal canal, a brain tumor, as well as a whole slew of other causes. And this is just a partial list for dogs and cats. Reptiles? We know even less what might be on the list of diagnoses.
As advised already, seek out a DVM ASAP, he or she might have something up his/her sleeve that could help. Sometimes vestibular signs (circling, falling over, etc, as described) in small animals can simply resolve on their own with only good nursing care as treatment. Keep your little guy as comfortable and as stress-free as possible, get a vet appt pronto, and keep us updated on the progress.
Good luck.
~Kerry