I suppose this particular screw also serves as an electric connection between the thread hole and the motherboard... Here is what you could do:
Take a strip of aluminum foil (according to the length of the screw) and wind it around the thread of the screw. You might wind it even two times, depending on the diameter of the thread hole and how strong it is worn out. Press on this aluminum foil to the screw so that it really gets into the thread of the screw and "sticks on" to it. Then screw it into the hole. It should now hold the motherboard.
If necessary, and when the screw is back in the hole, you might slightly fix it with superglue at one place (dotted), so that it can't move again.