Hi and welcome! I have an 8-year-old rattie who I adopted a year ago and he slept SO MUCH for the first few weeks I had him. I figure it was a combo of being older, exhausted from the turmoil of going from shelter to foster home to forever home and also the cold weather (it was February in NYC). He still sleeps most of the day (doesn't play) though really enjoys his walks--I give him at least 4 decent walks a day (apartment living means no outside space for him to go out and pee). I'm sure your little guy will come out of his shell soon and I think you'll find out what stimulates him, be it playing or walking or whatever. Thanks for adopting! Oh, and you talked about finding a good food--I feed my guy Innova senior wet food (he doesn't have many teeth) and I really recommend it or Solid Gold (but they're having shortages of their wet food--I don't know if you want to feed wet or dry).