Did you find him? I had ferrets many years ago and when they would get lost it was often because they got into a space they couldn’t get out of. Once the dryer vent tube. Inside the couch or mattress. Dark hiding holes. Ours loved raisins and maybe you can coax him out with a new smell. If he is outside, I am not sure where he would go. They are domestic animals and cannot fend for themselves. Again, check small holes and hiding places. Could he have tried to go back to his old home?