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.