Can you interchange System and Technic LEGO? My grandson would like to motorize his LEGO builds. Does the Technic motor pack only work with Technic LEGO or can he mix it with the regular sets that he already has? 

What should he start out with?
I assume by the Technic motor pack you mean this:



The straightforward answer is in my opinion "no", the pack does not contain enough pieces, nor do System sets contain enough unless they were explicitly designed to have moving parts and allow motorization (e.g. the carousel or other LEGO expert fairground sets)

You need "some" LEGO Technic to allow for moving parts. The LEGO Technic itself is then in many ways compatible with System Lego. 

The amount of Lego Technic your grandson will need depends too much on what he wants to achieve to give a generic answer.