Sorry to hear this. Have you put up signs, contacted animal control and rescues in your area, put an ad on Craigslists and alerted neighbors? Check the yard, under bushes etc, on hands and knees feeling around? Check in the late morning when he would come out to bask and warm up. Don't stop looking, many have found theirs even months air a year later. Also, if it gets cold where you are, he may try to brumate, so be sure to keep looking when it warms up again.
Good luck.
Russian tortoises are great little escape artists. This isn't a good time of year to lose a russian tortoise. He'll probably find a nice spot to dig in for the winter. I wouldn't be surprised if he hibernates all winter and you find him next spring in his habitat.
I wish you luck, but if you read over some of our "lost" tortoise posts, you'll find that most of them end well with the little tort found alive and well.
I recommend leaving out some of its favorite food and see if he/she comes for it. My only tortoise escapee was captured trying to get back in its pen for food.
In the meantime, do not cut your lawn with a lawn mower.....................