Well, he doesn't have parasites and he's still acclimating to his environment, so it's not time to panic. I would try giving him some other foods. Dandelion greens are a great choice because of their calcium to phosphorus ratio, and all of my reptiles seem to love them. Walk around the produce section of the grocery store and see what you can find for the little dude that you think he might like. Try to do some research on diet to find out what stuff is best. I think there is a website at www.beautifuldragons.com that is primarily dedicated to the care of bearded dragons. Though I question some of the care techniques, there is a very extensive nutritional content page that gives the content of many foods that are regularly available (as well as some that aren't), and a rating of how good they are for bearded dragons, which can easily be translated into how good they could be for russian tortoises.
Check the temperatures of the outdoor enclosure. Specifically the temperatures of where he basks, sleeps, and hides.
Other than that, if he has a clean bill of health from the vet, I would think that just giving him some time to adjust is a good approach. If you're really worried, you might think about getting a gram scale so you can keep track of his weight loss/gain by checking every few days. If you are still worried, you can always try taking another fecal into the vet (a good fresh one), because sometimes things can go unnoticed in the beginning stages.
Good luck and keep us posted. As long as he is eating something, just start by offering different foods as I suggested, and leave him alone as best you can so that he can adjust to the outside world. You should also try encouraging weed growth in the outdoor enclosure, because if you get enough dandelion, plantain, etc. growing in the enclosure you could easily have enough to sustain a healthy russian tortoise, meaning the little guy would be more self-sufficient.
Any new pictures?