Russian tortoises are "programmed" to eat when food is available. In the wild they probably emerge from burrows when weather permits, consume as much as they can in order survive long periods underground. Most don't stop that habit in captivity. Russian tortoises may be prone to becoming obese in captivity more than any species.
My RT will eat almost as much as you are willing to feed her. I call her the boo tortoise after my chowhound lab. I find that in her cage she will bask a lot but when I let her out to exercise she really explores the house, so I try to make sure she gets lots of exercise daily. The vet told me to make sure my floors are very clean before letting her roam though. He said they will try to eat anything they can fit in their mouths.
What is the bedding he is on?