How big is he? the smaller he is the more dire the situation. Your first concern should hydration. Get some water and electolytes into him. Second, make sure he has adquate heat and humidity. He should have a thermal gradient of 75 to 95, humdity 60-80 percent. He should have climbing space and a source of water large enough for him to get into completely. If you just got him you should give him some time to adjust before trying to get him to eat, maybe a day or two. Get a syringe to administer water electrolytes, and when he is ready for food, start with natural yogurt(active cultures, this will reestablish his microflora, and then baby food, beef, chicken. Once he is taking that from a syringe without much fuss move on to Zoophobas(superworms) and crickets and then to pinkies. Stress is actually a more serious threat to an adult then starvation. You can reduce stress by providing a proper setup as mentioned above and providing greenery for him to feel well cameoflaged, Limiting handling and force feeding as much as possible.