OMG nnikol,I don't really know,but I would try with some really smelly food,a can of people-tuna would do,not many cats can resist that.
But don't leave it there,try to entice him down.
If that does not work,you might have to call the shelter and they can get him out with a catchpole,which would probably stress him terribly,but he HAS TO come down.
Please,if you get him down,somehow cover that entrance up. Good Luck
