He has probably been developing a calcium deficiency because his feeders should be dusted lightly with
calcium without D3 daily or at least every other day
calcium with D3 2x a month
multivitamin 2x a month
It is very important to use calcium without D3 for most feedings because too much D3 is very bad for chameleons.
Calcium is not just needed for strong bones, it is needed by all of the muscles including the heart, intestines, tongue, etc.
When a chameleon is not getting enough calcium, his body will steal calcium from his bones in order to make sure that his muscles receive the calcium they need in order to work right.
I have bearded dragons and they are known as Teflon lizards for a reason---they are very resilient and easy to care for.
Chams are more delicate in comparison and they don't tolerate things that are not close to perfect for very long.
A calcium deficiency is often called MBD and here is much more information about it--written by a veterinarian who also owns chameleons
https://www.chameleonforums.com/what-metabolic-bone-disease-mbd-looks-like-how-happens-how-fix-95071/
You can put something soft on the bottom of his cage, such as a towel, to help keep him from getting hurt.
You can take him to a knowledgeable vet who can take xrays of his bones and draw a blood sample to determine his blood calcium levels.
A vet can give him an injection of calcium to quickly get calcium into his system.
For home treatment, you can give him 1 drop of Calcium glubionate syrup--the kind sold for people(calglucon syrup is one brand) in his mouth in the morning and another drop later in the day , plus the daily calcium without D3 on his feeders should have him feeling stronger soon.
As I said, a vet visit with a good chameleon knowledgeable vet, can give him
a calcium injection and then you will be administering the calcium syrup orally every day, too.
If basking temperatures aren't in the right range, he may not be absorbing enough of the nutrients from his food.
You can read the forum's Veiled cham caresheet for more on that, as well as much more good information
https://www.chameleonforums.com/care/caresheets/veiled/