Right now I would keep him on something simple such as kitchen roll, newspaper or repti-carpet (basically something that can't be swallowed and easy to clean).
Get tonnes of decor (fake plants, wood, stones) and as many hides as you can (even an old butter tub with a hole cut out). The more secure they feel, the more likely he'll feed. You'll also want a bottle cap full of calci-dust (no D3) in the tank 24/7 to allow him to self-supplement.
Do you have heating in his tank? If so what source, if it's on a thermostat and what temperatures are you getting? My fatties do best between 90-95F (depending on the individual). You also might want to consider getting a small UVB light, this will help with calcium absorption.
If you don't already, you'll need some kind of multi-vitamin (Repashy Calcium Plus is good) to dust his food with. Try to offer a variety of livefood, one of mine loves crickets and cockroaches but the other will only touch mealworms (again it's down to the individual and their requirements). Remember to gut load any food you give him.
Edit: His thinness could be caused by many things - incorrect heating, not enough food, but also illness and parasites. Correct conditions may only be so much help for him. When you're able to, I would strongly advise getting a faecal test (http://www.palsvetlab.co.uk/
These guys offer a very reliable, cheap and quick service - highly recommend them!), this will tell you if he has any fungal/bacterial infections but also parasites which often cause weight loss. Then get anything treated by the vet. I would also edge on the side of caution when it comes to judging his calcium levels - provide him plenty of calcium (and to a lesser extent D3) as bendy bones and disfigured limbs really are later stages of MBD and the likes.