My grandson wants a LEGO motor. What should I get?
You say he is looking for a "Power Motor", which sounds like he is referring to the LEGO Power Functions line. The bare minimum you need to get a spinning motor would be one of the battery boxes and one of the motors.

Battery Boxes:


AA Battery Box
AAA Battery Box
Rechargeable Battery Box (requires Transformer 10V DC)


Motors:


M-Motor
L-Motor
XL-Motor


(They also make a servo-motor and a train motor, but these are made for special purposes.)



The terms you listed however refer to the different generation of Mindstorms robotics.


EV3: Current generation
NXT: Previous generation
RCX: First generation (rather old and difficult to use with modern computers)
NXT-G: The GUI used to program the NXT robots.


Mindstorms do include motors, among other things, but are mainly geared towards use with robotics.