Make sure your husbandry is spot on. Give him a week or so to settle in and then offer the same food (live rat) he was offered originally. I would start small a weanling perhaps. It will ease his digestion back to working. A week after offer again but offer 10% of his body weight or so. When he is feeding regularly you can switch to F/T if you want. I would also suggest a vet visit for a check up. Even if he is healthy it will help to establish a base line for the future. Fecal exam too.