I can feel my rabbits spine and ribs?

im taking care of some rabbits for my inlaws and i picked one up because i was moving things around in his cage. thats when i noticed that i could feel his spine and ribs. he eats and drinks still, which is why i didnt notice ne thing sooner. he stays in his cage, and hasnt had access to any dog, cat or racoon feces that i know of. what can cause this? hes still active, drinks and eats well. ne ideas? thanks :)