I would first go to the store and check the setup if they have the proper UVB, even if there is one it might not be giving off UVB anymore as UVB tubes "expire" after about 6 months.
Then check what they are feeding the chameleons, are the crickets dusted with calcium or what?
After this if you see they arent using UVB and dont dust the crickets for them, then you its pretty safe to think that he has just got a normal MBD due to no D3 supplements, no UV exposure or both.
Then it is easy to get him to recovery, what I used to get 1 of my females to recover was TREX liquid calcium or bone aid I cant quite remember that name but its one of those 2.
Give the cham 1 drop a day and it should recover in a couple of weeks.
Then have a chat with the store owner or whoever does the reptiles and tell them their proper requirments for survival, but it seems that could be hard since you dont know to much about them either, maybe you should do alot of research and get that done, ay?
Good luck