Hello,
Do you have a picture of him & of your tank setup?
Please review your set up with us, such as your UVB lighting, temperatures, supplementation, etc, too.
He could have metabolic bone disease from previously poor care. Is he paralyzed or just doesn't want to move?
He might not be able to self regulate his body temperatures very well if he isn't used to a higher basking temperature. Or, he could have a respiratory infection also. Does he cough or wheeze, or appear to have any mucous in his mouth or nose?
You can also try giving some canned sugar free applesauce with canned unspiced pumpkin mixed with a drop or two of olive or coconut oil, daily to help.
Tracie