A russian tortises diet consist primarily of weeds. Dandelions are a favorite. If you put him outside and then went in to get food, he might of already eaten something like a leaf or dandelion while you were preparing it. Or maybe he just ate a lot back at the store before you brought him home.
Reptiles aren't like mammals. They can go a while without eating. And usually go off their food in a new place. Just
He's just not used to his environment. Put him in his tank and leave him for about a week, you can pet him a bit but don't pick him up or take him outside. Offer him fresh food and remove what he doesn't eat.
After a week or two when he's comfortable and eating again you can start to play with him.
Maybe he's not wanting what you've offered? What did the previous owner say was your tortoise's favorite food? It's also quite possible he's still adjusting to his new surroundings and will eat soon enough. Give him a day to get adjusted and then go from there.
Is winter, so probably it will be eating less. I wish i could help, my mom adopted an stray tortoise once, but the whole time he was on the house, he never ate anything. Turtles dont usually eat like dogs or cats, they sometimes take their time ( at least my water turtles been eating less since winter began ).
He sounds depressed/confused with his new surroundings. With any new exotic pet it's not a good idea to just thrust them into a daily activity full of sounds/sights/smells that they haven't adjusted to. They need time to adapt to a human/domestic household.
Also, as another user said, check out some turtle forums/websites that know much more about turtles than random deviants can.
He probably isn't sick. It's his first time in a new environment so he'd need to get used to it. This is probably off the topic of a tortoise but I have a Bearded Dragon who would hardly eat until the start of this month and we got him during the summer. Perhaps you could leave his food in his new home and leave him for a bit or something. He could take little bites after a while.
You should find yourself a tortoise keeping forum =/ there's forums for every type of pet keeping, I'm sure you'll get some better educated answers out of people who keep tortoises than the people on deviantart.