When a conductor is placed in any external electric field, generated by any source, then the net charge inside the volume of the conductor is zero. Why? Consider the situation when the electric field is "switched on". Then, as a conductor allows free movement of electrons, and thus _charge_ inside its volume, so the charges start moving inside the conductor.
Now, when will the charges stop moving? When another force stops them from moving, by opposing and cancelling out the force due to the electric field. Inside the volume of the conductor, in the ideal case, there will be no other force on the charge (in the form of presence, or lack of electron). But, when a charge reaches the boundary of a conductor, something changes. Considering a negative charge, or an electron, deep inside the volume of the conductor, the force due to the positive nuclei on the charge would cancel each other, but near the surface, all the postive nuclei would pull in the same direction. For a postive charge, this would mean a lack of electrons, which moves by drawing electrons from nearby regions. This cannot happen at the boundary of the conductor. All this would, simply put, prevent a charge from escaping the volume of the conductor. So, we see that ancharge in a conductor will keep moving till it reaches the surface of the conductor, where it stops.
Now, when the charges have reached the surface, why don't they move around the surface? Well, they do move around, till the field generated due to the other charges (accumulated on the surface) prevents our charge from moving even along the surface! When both kinds of motion (towards the surface and along the surface) stop, then we get the final charge distribution on a conductor. All this happens within a very short time of around few milliseconds.
Finally, if you take the specific example of placing a positive charge somewhere inside the conductor, the same reasoning applies, that is:
the electrons from surrounding regions are attracted to the positive charge,
the electrons from further regions are attracted to the newly created positive region, and so on,
- till the positive charge reaches the surface and there are no more electrons to attract from outside the conductor, finally making the postive charge reside on the conductor's surface.