I understand your need to "save" him.. trust me. My guy was dumped, LITERALLY, on the doorstep of a local vet (who happens to be an old friend). I rescued him from her despite having very little knowledge of chameleons (I've keep tons of other reptiles but never a cham before). I took him in mainly because I can't handle to see an animal in distress. I can't save them all, but I can save some - so kudos to you for doing what felt right in your heart, even if it doesn't solve the bigger problem of these stores housing chameleons and other animals in general.
Chameleons truly are magnificent little creatures, but the amount of care they require is definitely not for someone without the proper knowledge
OR the ability/desire to seek out the proper information. Unfortunately a lot of people that buy chameleons at these stores do so out of impulse and end up with disasters on their hands - a sub-par animal coupled with horrible husbandry techniques passed down to them from the store.. it's sad really. The point I am trying to make in this ridiculously long, and round about way, is that the pet stores that sell these animals should truly not sell them to begin with. Any person can walk in there today and purchase one - and that really isn't the way it should go. Recently while buying a few simple supplies for my guy at a Petsmart I happened upon their "Reptile Specialist" and had a conversation with her about "Chameleon Keeping" (pretending I had no knowledge of them what-so-ever) the information she gave me would surely leave any cham bought from them in rough shape quickly - she made no mention of calcium, only admitted that they needed UVB AFTER I said something about "hearing that they needed it", she told me they drink from standing water, never mentioned a need for misting, and said that a long glass aquarium is the best enclosure for them.. etc etc. It was really scary info and actually most of it was the exact opposite of what a chameleon truly needs. I'm willing to bet the pet store you got this guy from also gives out similar information, an assumption I am making based on the looks of that poor fella! (But I could be wrong.)
I'm hopeful that you can turn him around quickly. Lots of love and positive vibes to you and your new boy!
(And, yes, at the end of our conversation I told her that I was pretty sure most of her information was inaccurate and that she should check out this site to get more accurate information - she seemed kind of annoyed with the suggestion though! Whatever.)